There is an issue for this: https://github.com/nvaccess/nvda/issues/6624  It's priority 3 so may not be addressed in the short term.

One workaround is that if you select cells - either a whole row or column, or entire blocks or the worksheet with control+a, you can then copy those to the clipboard, and have NVDA read them with NVDA+c (you can read the selection with NVDA+shift+up arrow / NVDA+shift+s however if the amount of text selected is large, NVDA will only read the number of characters rather than the text itself).

Kind regards

Quentin.