maybe look at this?
not sure, but on falling block fall may be the event you need, unsure if that's called before or after falling
https://skripthub.net/docs/?id=4499
Skript Hub - Documentation
Called when an enderman places or picks up a block, a sheep eats grass, a silverfish boops into/out of a block or a falling block lands and turns into a block respectively.
event-block represents the old block and event-blockdata represents the new replacement that'll be applied to the block.