I really think this is one of those bad windows updates. I have noticed this week that things have been running a lot slower than normal. While I dont know for sure if that truly is the issue it would check out since the laptop you had was not connected to the internet and would not have gotten the update.
When you install Office 2021 even though it's the most recent install file did you also go up to file > Account > Update Options > Update Now?
Sometimes updates have to be manually downloaded like that. Let us know.