Yes, classic symptoms of read-modify-write.
So, now it's not a bug anymore - just sub optimal coding combined with capacitance on the output, high clock frequency and the hardware implementation of the PIC itself.

If the PIC has a LAT register (which yours do have) then use it instead of the PORT register for things that may suffer from RMW issues.
Code:
    LED5 VAR LATA.5
    LED4 VAR LATA.4