I've only experienced this issue since teleworking as there is a lag in the keyboard talking to the apps in the Remote Desktop environment.

I use ALT + TAB extensively at the office PC, however, it only works from my home PC environment rather than the Remote Desktop environment.

So, by habit, I press the ALT key often and below is how it gets activated:

To open the Research pane, do one of the following:
Press Alt+Click the left mouse key in the selected word, phrase, or name.

What was happening for me is that when the Research Pane was open, Outlook wouldn't let me type anything, even after I closed the Research Pane by clicking the X in the upper right corner.

The solution by this poster, ALT + rr worked for me by "releasing" the blocking of my typing in the Outlook message screen, thanks!