Would a exhaust leak cause the idle to drop upon startup? Could there be some sort of vacuum leak?

It ran fine other than being loud before new muffler
Upon startup, the idle has dropped to under 500 rpm. It has a hard time running and does stall after awhile.
Once warmed up, the idle returns to around 700-800 rpm. It runs fine then.
If shut off for awhile, it runs rough, but does not stall. Not sure on the rpms for this one.
Car was off the road for about two weeks.
The battery was swapped to different one until the original one charged on a Battery Tender. It is better now that the original is back in the car.
The stock muffler was cut off and replaced with a Monza Sport one after the cat converter.
It was bolted on and not welded.
Everything is tight and there does not appear to be any leaks. It sounds correct.
New OEM hangers & rubber donuts were used.
I did not see any sensors going into the exhaust under the car.
Oxygen sensors were not touched.
Nothing else was changed other than rear brakes being done at this time.
It has OBD 1 and no indicators are com