Why does PowerPoint sometimes open files "read-only"?

Sometimes when I open a PowerPoint file on my mac-mini (Big Sur 11.2.3), PowerPoint shows it as read only and won't save any changes I make without me saving a copy as a new file, which is a nuisance, although not crippling. When I check the file's properties with "Get info" it shows I have read and write privileges. If I reboot the computer, the exact same file opens up without the "read only" problem. Any ideas as to what's happening here?