NextZXOS freezes with if1 attached if the expansion bus is turned on
This is a known issue and is caused by the hardware unable to hide memory accesses from the expansion bus if the internal divmmc is about to replace rom. The reason is the decision to replace rom by the divmmc is not made until /mreq goes low which means the expansion bus will also see /mreq go low briefly before the internal divmmc does something about the memory access.
There is one cycle of delay at 28MHz before the expansion bus sees cpu signals so it could be that it will be possible to hide such memory accesses. Left as an open issue to remind about this possibility.