The FX is assigned to Effect Button II, the lower right led on Button I is not lit (it's just the picture).
The Lock is on the Input section (I use different saved presets for different guitars/PUs).
As said, the issue is random and on different rigs (not always the same ones, it changes when I restart the unit). To me looks like a Sync problem between IPad and Stage.
Do not know if it is relevant, but I always start first the Stage and then RM on the iPad, after it the Stage shows "Sync", sometimes only for a second, but some other time it takes up to 10-15 secs. Stage at boot is always in Performance Mode.
EDIT: I've never used Wah High Pass, not once, not even to try it