Possibilities:
There is more than one size stem hole, at installation they got it wrong, at repair they didn't catch it.
The problem might be a leaking wheel?
At each visit, work got assigned to the tech who doesn't know how to install valve stems? I would hope not, especially on a 'come-back.'
Stems have defective cores, or the wrong cores? There have been recalls of stems of other types because they came with bad valve cores.
Whatever is wrong, it is not what they think it is and trying to fix.
This is the kind of work I take to one specific local shop, because I know they do the job right and don't source parts from bargain suppliers. I don't have that confidence in most national or franchised service shops, not even those carrying the name of major tire brands, until experience tells me a particular shop is managed so the the job gets done right.
P.S. my tire pressures get checked before each trip, daily before each driving day. Sealing stainless steel caps on stainless steel stems.
