This happened to me a couple times during dungeon runs, so I decided to test on a training dummy and it’s replicated consistently.
After “consuming” the 50 stacks of Light’s Deliverance, if the free Hammer of Light is not used within the given time frame (12 seconds), the skill/action slot will not immediately return to Wake of Ashes. It will remain as Hammer of Light (but costing 5 holy power) for approximately 9 more seconds and WILL prevent Wake of Ashes from being used if it comes off cooldown during this unintended period, severely impacting the flow of the skill rotation.