I think you captured this point quite well. As far as what's normal/stable - this will differ. I've had the Orbi RBK50 for about a year and recently switched to a carrier that offers 400Mbps download speeds which was an upgrade from my previous carrier that had 40 Mbps as the max. During the time prior to my service switch, I too saw zero issues with my Orbi and it was what I would consider "stable."
After switching to the new carrier, I noticed that within 2 to 10 days after reboot of the Orbi devices, download speeds on all wireless devices (sometimes the satellites, and other times the main router) would never go more than about 45 Mbps while wired devices could easily hit the 400+ Mbps. A reboot of the offending router/satellite would again fix the problem for the wireless devices until problem recurrence.
I suspect the problem of devices getting into this state has likely always been there but went unnoticed because my previous carrier was capped at 40 Mbps. This one change in my environment easily exposed the problem with the stock Orbi firmware. Seeing the number of posts about this issue on the Orbi forums, I knew I wasn't the only one running into it.
It's only been about 10 days since I upgraded to the latest Voxel firmware and so far so good! However, from speaking with a couple of other folks that were seeing similar problems as me (in their case, problems would start within 2 days of reboot), they are either no longer seeing the problem or are seeing it only after 30+ days of uptime on the Voxel firmware. Thus it's definitely an improvement, and I'm hoping that I may be able to localize and possibly troubleshoot the problem, with help from the community here, should it return!