@koalb I may have gotten to my root cause.
I was using an older iPad Mini (gen 4) with TR and my Saris H3. When I used my older Wahoo Cadence sensor, no issues. Replaced the sensor with a new Garmin Cadence 2 and the spikes started.
My indoor trainer was getting my older devices, so I temporarily swapped in my newer iPad Mini (gen 5) and, after 3 hours of trainer time (2 1-hour sessions, 2 30-minute sessions, across 3 days) I had no cadence spikes. I suspect the newer Cadence sensor was expecting more current Bluetooth/BLE tech.
If you are US based, as it happens, Best Buy is running a promotion on 10" gen 9 iPads, for $269. They also offer trade ins. My gen 4 Mini was in good shape and I got $55 on the trade in, bringing the cost down even more.
I’ll know more in the next few days, after I ride with the new iPad, but the 3 hours using the other mini would point to this being resolved for me.