Had similar unknown blotches in my latest bake.
Ended up ignoring them after poking around for 4 hours to debug.
Chalk it off to a bad engine. That’s exactly What it is.
It’s not the model.
It’s not the materials or their reflectivity.
It’s not reflection captures or their resolution.
It’s not uv maps or their resolution.
It’s not light bounces generated by lightmss.
And well, the last thing one could test is removing the light. But then it would bake black, so ¯_(ツ)_/¯
One thing I did forget to check (installing the latest atm so cannot verify), was the light complexity on that specific spot.
Maybe if somehow 6 lights are intersecting one can expect for lightmass to get confused or render badly regardless of what engine is used.
Obviously same exact scene in other engines doesn’t bake any artifacts (however the light setups may differ by design, so its not like saying much).