It is NOT a bug!
Most likely it is MPSIM, which I never got reasonable results from.
Which is why I was so happy to find Proteus, which has no problem with your program.

It would make no sense at all for that to happen.

Your program only has 5 byte variables, so CNT is already in BANK0.
PORTA is in BANK0.
PORTC is in BANK0.
PIR1 is in BANK0.
The bank never changes while in the interrupt handler.

I'm affraid you are chasing problems that do not exist, except within MPSIM.