I opened the current ATP calendar to double check:
https://www.atptour.com/en/tournaments
And found out only 2 more 250 tourneys (Doha & Delray Beach, FL) having SGL 32. All other ATP250 have SGL 28. All others having SGL 32 are ATP500.
U r incorrect about Brisbane and Chennai and your "local" swing. Chennai is Challenger level not even listed in the calendar above. Rio is ATP500. Sao Paulo has SGL 28.
When Halle was ATP250, it had SGL 28, last time in 2014:
https://www.atptour.com/en/scores/archive/halle/500/2014/draws?matchtype=singles
I haven't checked the history of Queens or others (too much poking).
So, after that check, my question becomes broader: what's so special about three ATP250 tourneys (Doha, Marrakech, Delray Beach), that they allow SGL 32 in main draw, as if they were ATP500? Cannot be just organiser's whim, I think. Or maybe they are "in-between" 250-500 because they do not meet all 500 level standards sanctioned by ATP, while just providing a playground for a full draw of 32 singles?