That was between two Android phones, with receiving phone being "oppo" or some such, with stock firmware. Receiving phone would see incoming file request, ask user to accept, then error out with "unknown file", and no way to actually save it. I've sent files via bluetooth from lineageos to lineageos, no problem.
I could totally see it being something stupid in how Android manages file associations or something. The default file managers tend to be fairly crippled and only actually show a subset of files present.
