Construction/ Home question. Help with fireplace/mold smell situation

If anyone could help me with some advice, or point me in the right direction, it would be greatly appreciated.
We built this house through a contractor in 2015, and reaching him for help with this is not possible or an option at this point.

Anyways, I have this natural gas fireplace in the living room. We haven’t used the fireplace in years and it’s maybe been used 5 times during winter months since construction. I found these photos from construction and uploaded in case it would be helpful to someone with knowledge of this type of situation. Last year during the summer, we noticed a sort of mold smell near the natural gas fireplace. On the other side of that wall is a guest bathroom that has no external windows. The bathroom cabinet which is back to back with that fireplace, stinks the mold smell. I tried asking a plumber, AC guy, Mitagation worker, if this is something they could look into and they all gave the run around saying that isn’t something they’d deal with. This summer it has returned and is an even stronger smell.
Inside the attic, above this fireplace rectangle column, there is OSB board nailed down blocking the opening. Not sure if that’s an issue.
My thoughts are that maybe the cold air conditioner air inside the house is inside the natural gas fireplace, and on the inside of that rectangle column is sealed off hot air. So maybe the fireplace metal is sweating inside of that brick column and making condensation inside that rectangle? I have no clue but something definitely isn’t right.
Would anyone in construction have any advice? Should I remove the osb board above it in the attic and open it up?
Who would I contact for help with this situation?