Intended mechanic - they wanted to added, it was working for a while, and then they broke it in a later change.
"Paladin seals will now persist on the Paladin for a very short time after they’re replaced by a different seal.
- Developers’ notes: The concept of “seal twisting” was only possible due to spell batching. With a greatly reduced batch size, we’re adding a slight buffer when you swap seals to allow you to proc two in the same swing. This was not an intended interaction in Original WoW, but it’s a fun quirk of WoW Classic gameplay, and we want to allow Paladins to retain the ability to do this. Please let us know how this compares on the PTR to how seal twisting currently works on live realms."