Off-by-one error in the forge is the best explanation I've seen yet
I would guess that there was an attempt to suppress procedural generation around the bubble, but they got OR and AND mixed up, e.g.
Suppress PG if |x| < 1.5 OR |y| < 1.5 OR |z| < 1.5 instead of |x| < 1.5 AND |y| < 1.5 AND |z| < 1.5 (the actual bounds seem to vary by star type.)
And I suppose it wasn't applied to stars on the main sequence. It might be interesting to see some of those star types.