I had the same issue especially when using OneNote where pressing space was causing the command to do an 'Undo'.
I manage to fix my issue by going into 'Settings > Accessbility > Keyboards > Full Keyboard Access > Commands', and from there in the 'BASIC' section, I removed the 'Space' in the 'Activate' option.
I'm using a Smart Keyboard with a wireless mouse.
Hope this helps most of you!
IOS v13.5.1 on an iPad Pro 4th Gen -