Well, in an effort to try to nail down if it is a controller issue, I’ve now tried two different controllers with two different receivers in multiple USB ports. They all show the same thing when watching the raw values coming off the controller (completely steady with no bursts of lower X-axis values), and every combination had the same problem in the game. I have not seen this issue come up in any other game, including Forza 6: Apex. This appears to only happen in Forza 7 and only when driving, it never occurs in any of the menus.
Next steps: I’ll reinstall Apex and FH3 and see if I can recreate the issue in either of those two games. I’m also going to try both controllers with my XBox and see if they have the same behavior there. I’ve played a few other games in the interim with the same controller and had no issues.
FWIW, I believe that I ran the Endurance race at Spa before the patch came out and, as mentioned, did not have this issue.
EDIT 1: Played a few rounds in Forza 6: Apex. Had no issues. Space is limited on the SSD, so uninstalled that and installing FH3.
EDIT 2: Did a run from Surfer’s Paradise out west past the northern side of the airport, then looped back on the highway to the south and back to SP. Lots of long straightaways at high speed, no issues with steering.
EDIT 3: Attempted to run some laps on Forza 7 with everything else on my PC disconnected (Saitek X52 flight stick, Fanatec Clubsport pedals) and I am no longer having the problem. I had steering assigned to the disconnected wheel. My theory, especially since F7 seems to enjoy reassigning my pedal inputs to a non-existent pedal set occasionally, is that F7 helpfully reassigned my steering inputs to one of the pedal axes. Since the pedal naturally is at the low end of its throw when I’m not pressing it, every once in a while it’d notice a little jitter out of the pedal and think it’d activated, then assign that value to the steering, then almost immediately notice that the controller had sent a new signal and switch back. Similar to what some other people have mentioned, how the menus will flutter between showing keystrokes and buttons to navigate, I think it’s really quick to think it’s found a new device and switch to it, then switch almost immediately back.
While I’m not thrilled about the idea of digging around to disconnect my pedals and flightstick every time I want to play with a controller, I’m happy that I think I’ve solved the issue, at least for myself. I’m not sure it helps OP, as I’m not sure having multiple devices connected to an XBox One is nearly as common as it is for a PC.