Seems to be fixed in XnView MP Windows Version 0.99.1 64bits (Sep 23 2021)
I'm able to once again use SPACE to tag files in the file list, and in fullscreen mode.
SPACE is used by default, you need to change settings>interface>keyboard - SPACE
Please make it so SPACE can be used to tag images again. I used that all the time. Now I can't get it to work no matter what I pick under settings>interface>keyboard>SPACE