I have managed to spend a little more time looking at this so here is my report:
1. Created a copy of an album.
2. Renamed all files in the format <Artist> - <Album> - <Track#> <Title>
3. Removed all tags from all tracks.
4. Clean install MusicBee 3.5, patched to ver 3.5.8699 P, and pointed at this one tagless album.
Filenames are displayed in the title field. All other fields remain empty as can be seen here:
On right-click > Edit, the filename is also displayed in the title field.
On checking the Tag Inspector there are no tags saved.
If you 'save' the tags at this point, the filename does get written to the title tag.
My comment would be that it is not what I would have expected but it does appear to be intentional.
Is it desirable? To be discussed (maybe), but I would understand an argument for this if only to display something (so you have a idea of what is there, or at least that something is there).
Using the Computer Node to view:
Editing the filename and then refreshing causes the displayed 'title' to be updated with the new filename.
I have been entirely unable to reproduce what you are seeing with the Artist field being populated, no matter what I tried with filename structure.
This is not a 'feature' that can be enabled/disabled.
I believe this behaviour is intended.
I cannot explain why you are seeing the Artist field being populated (other than maybe you are using an older version of MusicBee).
Further (rhetorical) question:
Where on earth are you getting all these files with no title or artist tags?
If you feel strongly you could post a wishlist request to prevent this behaviour. (Note that this is probably a niche case - I have not noticed this until now and I don't remember any other similar reports.)
Other than that you may just have to come up with a workflow that checks for this before sending to your library (are you using the Inbox?)