Bronco wrote:

My hypothesis is that they built the frame openings too large and
shimmed too large of a gap, resulting in the eventual warping around
where the shims were placed after tightening over them and now that the
frames are not square, no matter how many new sashes they put in, they
are eventually going to warp.

I have been told that when installing vinyl windows there is a risk in
using expanding foam to insulate the gaps around them. Normal expanding
foam can continue to expand over time, exerting significant force and
bending or warping the vinyl frames. There is a limited-expansion foam
for use in that application.

Just a random comment; hardly a diagnosis, but an example of what can
go wrong with installation.

Chip C
Toronto