Comment by piker

Read the article.

What happened is exactly what it suggests. OneDrive migration doesn't just copy your files to a server. While the migration is occurring, the operating system is optimistically re-mapping them all to a cloud URL and/or deleting them entirely.

Halting the migration and disabling "backup" returned the mapping back to the local disk, and I was able to open the file as normal.

I know, it's as crazy as it sounds and your bet was my bet at first as well.