There is nothing separating loaded saves from 0.20 and 0.21. Both go through the exact same patch routines upon loading, and they couldn't freeze the game anyway since they only set variables. Perhaps your saves from 0.21 were made in places that became incompatible with something in 0.22? There is always the risk that something can go awry when updating Ren'py games if the engine is unable to properly locate the place where you saved at.
It might, of course, also be a problem specific to the Mac version. Since I don't have any Apple hardware, it's impossible for me to test it.