Man KWAN
In this case, if it happened yesterday, we’re talking about a bus B263 operated by TPF, the same company operating the Bulle-Montbovon railway on workdays.
Like a funicular, a bus has also just a single class (2nd) layout, but your Le Paquier-Gruyeres ticket is a 2nd class ticket anyway.
Now were you really insisting on a 1st class seat while boarding the bus, maybe making a scene? (To be honest, that could explain the driver’s behavior…)
But anyway, as we were not there witnessing the situation, the exact reason why boarding was refused needs to be clarified / figured out with TPF customer service, as SBB are not supervising or instructing those drivers in question.