I haven't seen this before... on iracing I was in the W13 mid race and my shifting would get rejected. I thought maybe it was a gearbox issue in the simulation after being hit from behind, retired the car, tried in practice and one point I came to a full stop in 8th gear trying to shift down. Never did. I even took a video of my gears stuck in 8th and shifting down nothing working.
It doesn't feel like a button issue, worked great just last night, but rather software crossed signal issue or something. Not sure though!
Here is a link to the vid.