I am not keen on the way the calendar works either, I much preferred WRC Generations where you were given a selection of things to do on a date and nothing really clashed. Well apart from the fact I couldn't fix the condition on my car for half a season once lol.
I haven't got far enough up the meter to encounter the bug you mentioned but I will keep an eye out for it once I get that far.
I think because the extra events in Generations were just one off stages usually, probably a lot easier to avoid clashes than trying to squeeze in entire extra championships.
I think it would be better if for the next iteration/major update/game whatever they do, to scrap to calendar idea and just let us choose our team members with a budget, have 2/3 slots for championships and occasional one off invitationals that we can do. So rather than having to fit everything into a linear calendar that can clash with everything, just let us enter what we want to enter at the start of the season, and then do it so we could have, Event 1, then WRC, Event 1-2, WRC, Event 1-3, WRC, Invitational, WRC, Event 1-4 etc. Then when WRC ends, have a 3rd championship in the off season. Just as an example.
And ditch the benefactor, cool idea, kinda pointless, split career into 2 paths like the F1 games, driver career and team career. Driver career you drive for real teams and what not, team career you have your own team like it is currently.