Formula bar gets auto activated when using alt+tab
If I am understanding your question correctly, it is because a key on the keyboard was pressed other than Enter/Return. Same thing happens if you press Caps Lock.
It is annoying at times, we must ignore the standard Alt/caps/numlock, scroll lock/tab etc. Its a candidate for Idea exchange.
May be you can think this is way beyond our thoughts but still you can overcome by using Key function Win + 1, Win + 2 for Windows users. I mean pin your Anaplan Window in the tool bar section (1st) then pin your excel or most wanted item as 2nd in your tool bar section then switch around the windows using the above key function.
I hope this at least give some work around 😉