Wow that's wild. This will be a fun one to investigate. This is definitely something I didn't test.
I don't know if you want all this feedback, but here goes...
In both versions of the mapper (current release and the beta you sent me), the tap key (still assigned as Q) works
while holding a modifier, but it definitely interrupts an alphanumeric key. So to elaborate, if I hold ALT and tap the FN key, ALT stays pressed and Q is also sent at the same time. But if I hold any letter or number and tap FN, you see the letter/number shut off briefly and Q activates, then goes back to the letter/number.
I'm going to try testing another modifier as the tap to see if it's something specifically related to the CTRL scancode...but I've not got much time...gotta get ready for midnight mass....
Edit: Ok, so, I tried random keys, it seems as though the tap function works fine with alphanumeric and backspace...however when I try the L/R Shift, L/R CTRL or ALT...no go, it registers another key IF it registers.
Last edit (for now): I tried moving the FN key to a different position, same results.