In mqx 3.3 to 3.5 _mcf5225_int_unmask (also _mcf5222 and mcf5223_int_unmask) does a bit-wise AND with 0xFFFE to clear the LSB.
Unfortunately IMRL0 and IMRL1 are 32-bit registers, so the upper 16 interrupts get unmasked on every funtion call.
I think you are correct. I'll let our FSLMQX developers know about this.
Thank you for mentioning it.