I have an object. Due to some reason, I have delete portions of the model. For deleting, I have to go down to the lowest level, hide the polygons to delete, then delete hidden. After that I loose the finest details of the uppermost level, though the level exists…It’s a big bug to me.
“edited by Moderator”
See TVEyes bellow 
I don’t think that will work Aurick. The morph target would be stored using a different vertice count and order compared to the model with deleted portions.
The workaround has been to hide the polygons you want to delete at the lowest SubD level, apply a mask to the entire model and then go to the highest subdivision level and back to the lowest subdivision level. Then it should be safe to delete hidden.
Since you have to access the highest subdivision level for the mask to register I would clasify the behaviour as, maybe not a bug, but at least counterintuitive.