The new renderer override and template used for Boost changes the classname of fake blocks from .fake_block to .fake__block (with a second underscore).
It does this as it ignores the class set by the old code and builds the class in the template from
and "type" in this case is "_fake".
This broke my theme CSS that targetted fake blocks, though I couldn't find any core CSS that relied on this class.
I did find one bit of core JS that seems to expect that class though:
It seems like it's intended to stop drag handles being put on fake blocks.