Currently Seal of Command is bugged, in a manner where the calculation of whether the spell will actually trigger or not (The PPM Calculation) is being delayed until the next Spell Batch which follows the initiating White Swing, this scripting interaction is incorrect; the calculation of whether the spell should trigger or not should happen instantaneously as the initiating White Swing occurs. The reason this bug is currently occurring is more than likely due to the “Spell Batching” (Spell Prioritisation) System which has been implemented throughout classic, and as a result the procs calculation is being pushed back to the next batch incorrectly, leading to scripting which does not accurately reflect how the spell should function.
Luckily when looking at Seal of Command we know exactly how it should function in relation to spell batches, because we have countless videos of Seal Twisting a technique employed throughout The Burning Crusade by paladins, where the player would “Twist” Seal of Command around a spell batch, in order to gain the benefits of two seals off of one hit, a mechanic which can be seen in the videos attached at the bottom of this post.
The method through which Seal Twisting worked was through a player would wait for their character to perform a White Swing with Seal of Command active, as the White Swing occured the server would then instantly calculate whether Seal of Command would proc or not, at this point before the next Server Batch took place, the player would then swap seals, this meant that when the server batch actually happened it would check the players buffs and recognise that you had a seal such as Seal of Righteousness or Seal of Blood active, and as a result apply the relevant damage effect of that seal in addition to the initial White Swing… At this point however, the Seal of Command proc had already been calculated and confirmed by the server, and so your character would then perform Seal of Command, at which point the server would notice again that you had Seal of Righteousness or Seal of Blood active, and as a result apply the relevant damage effect of that seal… in effect 4 attacks from 1 swing.
This is a documented and undisputable mechanic which existed throughout both Vanilla and The Burning Crusade. Unfortunately however this is not how Seal of Command works in classic, and so the spell does not work accurately at all. While you should be able to swap seals following a white swing but before the following server batch and reap the rewards of Seal Twisting, in its current state and scripting; this does not occur. Because the PPM calculation of the spell has been pushed back to the batch following the initiating white swing, this means if you change Seal, cast Judgement, or are dispelled prior to the spell batch parsing then your character will never perform Seal of Command no matter what, an interaction which does not reflect the original game or the recorded documentation we have of it. What this also means is that paladins are currently able to retroactively apply Seal of Command, meaning that if a shaman purges a Paladin right before they swing, the Paladin then swings, and then following the swing the Paladin activates Seal of Command, the Paladin will still hit the Shaman with Seal of Command regardless of whether the seal was active at the incident of the swing, another interaction which should not take place.
Furthermore based on the principles of Seal Twisting, if a paladin Seal Twists Seal of Righteousness off of Seal of Command before the Spell Batch following a white swing, the following interaction should occur:
A White Swing hits a target with Seal of Command active, before the next Spell Batch the paladin activates Seal of Righteousness, Seal of Righteousness hits the target (from the initial White Swing), Seal of Command Procs, Seal of Righteousness hits the target (from the Seal of Command melee Swing).
e.g [White + SoR + SoC+ SoR]
Currently however due to the Proc calculation being pushed back to the next Spell Batch all that happens is the player hits the target with a White Swing and Seal of Righteousness, and the Seal of Command hit never occurs.
In conclusion, based on actual footage of the game; and first hand knowledge of using the mechanic, we know that swapping seals or casting judgement instantly as a white swing hits the target should not stop a Seal of Command hit from occurring, in fact it should enable Seal Twisting. However, currently; due to the calculation of the proc chance occurring during the following spell batch, the spell is not performing in the same manner in which it functioned in the original game, and so is not scripted correctly. This issue will only compound with the release of The Burning Crusade, as Seal Twisting was a defining part of it for Paladins and an extremely effective method of boosting their dps, and due to the bugged way in which it functions currently; this mechanic will more then likely not function in the future.
Please fix this bug as soon as possible, the original Blizzard Devs of vanilla WoW have already announced that the original coding of Vanilla featured hard-coded script resolution for some mechanics i.e scripts which ignore spell batching and are resolved instantaneously, the Proc calculation of Seal of Command is one of these hard-coded scripts. This bug has been present since the start of Classic WoW and affects paladins in a fairly major and detrimental way.
Examples of How Seal Twisting is performing currently in Classic:
Seal of Crusader into Seal of Command Twisting - Retroactive Seal Application
Seal of Command Post-Hit Twisting Combinations
SoCr into SoR Twisting - Retroactive Seal Application