Hi,
I experience a strange issue whenever I am building within my settlements (which I do a lot).
As soon as I build somerthing using the workshop interface (bound to third mouse key in this case), I sometimes (not always but often) cannot use the Pipboy Interface by pressing "M" or "TAB" (afterwards).
I usually want to fast travel after doing some workshop stuff, therefore it is quite apparent.
I have also found a workaround: If this happens I can press "ESC" to return to the system menu. If I leave this menu, the TAB and (in my case M) quick-keys are working again.