Arial Black
Adventurer
Disintegrate's ash effect has already triggered. Gaining hit points later does not undo that as there is nothing in the spell to say that it does. Without being able to show me where it explicitly says that disintegrate's trigger is undone by reversion, it does not get undone by reversion.
When the druid reverts because of damage sustained, there is no 'gaining' of it points!
The beast form lost all, and the druid form lost more. At no point during this process did the druid gain hit points.
Disintegrate never triggers because the damage from the spell causes the druid to be reduced to 27 hp, not zero.