The blocks that get the Illagers summoned in them don't replace themselves with air, but the block that was possibly on their place, lower floors get for example grass, stone, dirt, etc. where the top floor always is air.
I know it's those blocks, as the Illagers spawn IN those stone/dirt/grass blocks.
Perhaps this is fixable by saying, always replace me with air?
Seed to reproduce (coordinates in the screenshots): -7727920473720483005
Note: it might be usefull to set them to NoAI upon teleporting.