This is definitely not an issue with DTM functionality.
This seems to be just how Floodlight tagging works. Searching around on the internets, I found that a lot of others have asked this same question about floodlight tagging (regardless of how it is deployed, e.g. GTM vs. DTM vs. directly on page). I could not find an explicit, definitive, or official answer about why it does this, though. But I did find some interesting theories, namely in this thread: