Hi, Peter

just some reading ... from that F... Manual, $ 5.27 :

Since this command automatically sets the data-direction of the pin it acts on, the Pin parameter should only be a PORT or GPIO register (or an alias to a PORT or GPIO register). If the command is directed to act upon a LAT output or a bit within a variable or SFR, it will attempt to set a data-direction register that doesn't exist. The result may be unexpected behavior since a bit is changed in a seemingly random memory location. This can be very difficult to debug.
B.R.
Alain