That sounds like a hack to allow programming an EEPROM or Flash module in place in the machine. Is your IIfx SIMM built out of ROM chips or EEPROM or Flash chips. Hmmm. Must be one of the latter two, as ROMs don't have WE pins. Even if it's ROM chips, I guess it's possible they designed the circuit board for a proto-type reprogrammable module and then used that same circuit board for some of the production ROM SIMMs.
My guess is that when they want to reprogram the module, they'd write to the ROM SIMM and every address would be the normal address plus the A19 bit, which would give them WE high. Wait, that doesn't make any sense. They'd have to hold A19 low to enable WE. Hmmm. So any time they read from the ROM without asserting A19 the chips would treat the read as a write operation? That's not good. Weird. Maybe there was an inverter on the module?