Turns out I was wrong about this bug not being related to the Autopilot Set Speed speed limit offset setting...
Even though I have the Set Speed setting toggled to engage at the
current vehicle speed with no speed limit offset, my car is still using the "greyed-out"/disabled speed limit offset setting (which for some reason I had set to +30mph) and applying that offset to automatically change my set speed on almost every highway on-ramp that I take. (The speed limit offset value is still retained in the vehicle settings even after you toggle back to using current speed. You can still see it in the Autopilot settings menu -- it's just greyed out.)
I verified this by changing the speed limit offset setting to a bunch of different settings, and the set speed reliably jumps to whatever that offset speed is on the vast majority of on-ramps. This happens equally whether the offset is set to a MPH offset or a % offset, and it also happens whether I leave the offset setting engaged or toggle back to Current Speed.
So it's really like 2 bugs in one:
- It's automatically increasing the set speed in the middle of on-ramps, which it's not suppose to do. (Note that TACC/AP can automatically decrease the set speeds when driving through interchanges and then later increase it back to the original set speed, as described in the owner's manual. But it's not supposed to randomly increase speeds based on speed limit offsets on on-ramps.)
- It's using the speed limit offset setting, even though I have that disabled.
I do still wonder if this bug is somehow unique to my vehicle, or if this is a widespread bug that's just flying under the radar since it's unusual to engage TACC/AP on on-ramps.