You might be on to something there. Mine have been broken for about as long as this thread has been open. Yesterday morning I noticed a tab I had open on my iPad appeared on my phone. I closed it from the phone and it went back to being broken.Apple One: I moved to this service, but the problem happens to my colleague as well, and he doesn't use Apple One. So it seems, it's not related to this service.
In fact, the problem is kinda weird: I asked him if he had issues with tabs sync, he told me, "No it's fine".
On his devices it was all synced well from iPhone to iPad to Mac. But, as soon as he closed a tab remotely. All was broken, the same problem we have. The tabs disappeared on all his devices.