@Chris_Clark1968, we have now experienced this also, and in researching for answers I came across this post from 2020: Failed to upload your file MS Forms - Microsoft Community Hub
So it looks to me like Microsoft Forms has either "intermittent issues" like this or will always, after some period of time since creation of a form, lose its "glue" to the back-end storage (OneDrive). I suspect the latter. I have reported the issue (as others probably will have) via the Office admin portal -> "Service Health" -> Report an issue.
Note that the post from 2020 above includes a nasty workaround (the last post currently) of copying a form and using the copy. This is because it appears that "new" forms don't have the issue. However, since all "new" forms eventually become old, I reckon the issue will arise again in time. Perhaps it's an expiry of some client secret setup that is supposed to be automatic (as well as hidden)?