Pianotech

Matteo, 

POSSIBLY the fact that he may be unintentionally destabilizing your tuning...have you ever actually watched him use the hammer?  Amateur tuners tend to move thi is around A LOT!  Just sayin...

Any chance there is an air current anywhere that might be affecting this section? Lights...heavy breathing...something warm on the plate...etc. Any temp change will do it, and if it just happens that the SB has extra heavy DB in that section (hard to determine I know) it will be more sensitive to any change whatsoever.  Of course recording studios want greater stability than most other venues. Do they turn the heat down at night? 

Just some thoughts to consider 🤔 

P.S. Add a little TSI-301 to your CBL...

Peter Grey Piano Doctor 

My feeling is that restringing will only add to the problems, since new strings take a while to stabilize...

I agree with Peter's suggestions. Especially with a less experienced tuner working on it... no snobbery meant but we know how much less we have to fiddle around with it with practice. In addition, how's hammer to string mating? If that's not great it can make it your window of tolerable unisons rather narrower.

I would measure downbearing too. Too little can also make notes hard to keep in tune, although you'd probably have heard it too.

I would put a few drops of thin CA glue at the base of each unstable tuning pin.

First check the torque of every tuning pin for all of the unstable notes as well as random notes all over the piano.

Second- place a data logger in the piano and get measurements every 5 minutes  24x7 for 2 weeks and look at the graphs

Third- check the hammers for mating and shape as well as condition

Fourth-confiscate the guys tuning hammer as well investigate who is doing the recording and what is being recording

Fifth- how tight or loose are the action centers and how good or bad is the regulation 

we have a number of performers here who beat the tar out of the pianos thinking louder is better- some play with the sustain down at all times