Condensation from chimney through walls?

[LedSomerset]

We purchased our 1928 house in May. At the time we knew some work had been done to fix a ‘leak’ above fireplace and at foyer area (backside of chimney). The work was said to be complete and corrected the problem. This house has plaster walls and since we turned on the furnace (oil burner with old radiators all over the house) we have noticed a gradual change to the repaired walls - discoloration and flaking. Also seemingly a bit damp. I am thinking it wasn’t a leak but some issue with flue / chimney that wasn’t rectified. Is this a condensation problem due to the boiler? It must share the chimney with our fireplace. What type of expert to call on this issue? I don’t trust whoever worked on this most recently. Brick house with plaster and lath walls. Technically one story I guess but the attic was made into another floor. First pic above fireplace and second pic on other side in foyer. Last is a detail shot in foyer. Thanks in advance for any observations.

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