Thank you all for ideas. I know she is likely just old with LOTS of things going on/shutting down. But good to think through things.
Recap of worm history to address some of the comments/suggestions:
She was regularly wormed, but found round worms late winter/spring on first investigation of weight loss. My understanding is the round worms are actually common in old and immune-compromised horses. We had been doing Ivermectin so switched to Strongid when we wormed at that point. Did stop weight loss for a little bit, but she did not start gaining weight.
Checked fecal in late April, was clean. Did worm again with Strongid (same as all other horses) just to be sure.
Found some eggs in check few weeks back in fecal (but not huge number and no adults observed). Switched again and wormed with Pyrantel. She was in same paddock, so not really too surprising that she was reinfected. Other horse in there has never been wormy (although obviously exposed). Vet thinks (and makes sense to me) that worms are a symptom and not cause of problems.
Between that and antibiotics, also concerned about what this is doing to her gut flora. Have also been doing probios after worming and will again after round of antibiotics are done.