So, have you tried a free-field measurement yet (aka not in the chamber)?
Antenna test is intricate. It’s hard to truly say what is or is not the problem here because there can be dozens of things that cause something like this, or it could really just be that.
Just looking at it, given the frequencies you’re talking about, if an engineer came to me asking about this and what to do next. I’d suggest a free-field measurement. In general, my comments would be that you’re testing at a low frequency (250MHz) has a ~4′ wavelength. In order for the fields to enter into the antenna, you’re going to want other things (walls, absorber, etc) at least that far away from it unless you’ve specifically verified the test setup, that you have absorber behind the antenna to prevent reflections, but far enough away to not artificially couple and affect the antenna. There are a lot of things much closer than that at the frequencies your talking about. Also, the chamber could also end up acting more like a resonant cavity than a chamber depending upon size and exact geometry. Again, maybe not a problem, but could be. Also, have you ran S11 on the antennas post install? Are they being detuned, or a reflection coefficient issue created during install? And so on.
The bottom line is that from the pictures, given the issue that you’re seeing, you could spend days/weeks/months tinkering with the test setup and never hit a definitive conclusion on the issue, as there are potentially many, and they could interact in complex ways. So, I’d tell my engineer to check S11 an as-installed items, and check it in various chamber configurations. S11 open-air vs chamber should be the same; if substantially different, something is up.
Then separate out the test chamber from the measurement, and then add pieces back in. Go find some open space somewhere, and just run a real quick simple test. Collect that data, and then start introducing your test setup piece by piece, verifying that you’re good every step as you do so. You’ve already half did that with the test from a certified house, but it’s just hard to truly say you’re replicating or there’s an antenna problem, etc when all we really have with these exact items is a somewhat problematic test chamber setup.