Them ending up on the DFA list is intended. It's getting more and more typical for players to not even play in the minors in their draft years especially pitchers and so to replicate that, the AI may move recently drafted players onto the DFA even multiple times.
I mentioned in another thread, but you look at some real life examples, you have a gap where players may sign, but are not actually assigned to any team. You can even see this with players who have already signed at times. That's more or less what this is simulating.
You can see that gap for Levi Sterling last year, between July 25 and August 5. Then for him, even once he was technically assigned to the FCL, he never actually suited up on the roster or played there. So this is really just replicating this real-life behavior.
https://www.milb.com/player/levi-sterling-815552
Now, that being said, it's probably not desired behavior for the AI to put most recently drafted players on the MLB roster (separately from putting them on the DFA list), so if that's happening and it's not a top pick who is almost fully developed, that might be something we need to check out.
If you have a league where you've seen this, please do post a link to the league files and let us know the specific players on the MLB roster that seem off.