My long-term disability insurer wants me to do an "independent" medical exam. Should I be worried?

I've been on long-term disability through my employer's policy (administered by a major insurer, won't name them but you can probably guess) for about 14 months. I have a legitimate diagnosis confirmed by two specialists - a spinal condition that genuinely prevents me from doing my desk job. I've been compliant with all treatment, submitting forms on time, the whole thing.

This week I got a letter saying they want me to submit to an "independent medical examination" scheduled at a clinic about an hour and a half from my house. They framed it as routine but this came out of nowhere with no explanation for why now. The exam is in three weeks.

I've read enough here to know these IMEs aren't really independent - the doctors are paid by the insurer. But I also know refusing to go is likely grounds for termination of benefits. What should I go in knowing? How do I protect myself? And is there anything I can do in the three weeks between now and the appointment to prepare?