@Alan-Kilborn said ,
I downloaded Notepad ++
You mean we weren’t actually starting with a Notepad++ related problem from the very beginning
I chose to interpret that statement as “I re-downloaded and re-installed Notepad++” (which could have had the effect of fixing the .txt file association back to Notepad++, so that they were properly icon’d for Notepad++ rather than Excel). Otherwise, … yeah, that statement gave me pause, too. :-)
But ignoring that, I don’t often have my OneDrive folders showing files that it hasn’t downloaded to my machine, so I don’t remember how seamlessly that feature of OneDrive works; but maybe Notepad++ requests to open the file differently than some other common applications, so OneDrive doesn’t recognize the request and download the file promptly enough; or maybe Notepad++ isn’t patient enough while OneDrive processes the request, and if the OP tried to open the file a second time, it would be there. But i think “Always keep on this device”, and being patient once, should help the OP’s current problem.