Does the cupboard have a door? It could be holding the smell because it doesn't have airflow like the rest of the room.
The door was off for over a year, but is in place now, it could be part of the problem though, yes, maybe some ventilation holes in the top of the doors will help...
You want airflow, I'd run a fan or air purifier in there 24/7 for a week or so and see if it helps.
@RodSaetan started
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Does the cupboard have a door? It could be holding the smell because it doesn't have airflow like the rest of the room.