Just curious if you're entering the formula in the cell or the forumula box once the cell is selected, or have perhaps tried both?
NVDA is not hanging for me, but, it had might as well be. Whether in the cell itself, or the formula box, if I type the equal sign then start typing the word, sum, I'll get an announcement for equal sign but then everything goes silent. I don't have letter by letter announcement as I type turned on, but even after the formula is complete (or even partially complete) and I hit space I'd expect what's there to be read, but it's not.
In the formula bar itself the behavior is the same when initially entering the formula, but if you revisit a cell later, and throw focus to the formula bar afterward, the formula is read correctly.
Also, if I start entering equal sign and have typed the S and U of sum, and begin down arrowing through the dropdown list of summation functions, these are not announced. I always get equals su (the letters I've typed, rather than the thing in the dropdown that has focus).
I'm using Excel 2016 and 2020.beta4.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
The depths of denial one can be pushed to by outside forces of disapproval can make you not even recognize yourself to yourself.
~ Brian Vogel