So the way I encountered this bug is as follows:
- I had a big lead so I decided to BM and finish backpedaling
- the lead wasn't as big as I initially thought, so I got passed or was about to be passed by rival
- I also got Boo'ed just as I was about to cross the finish or just after I crossed the finish

When I continued to score tally, it went back to being normal as if nothing ever happened. Same goes for the rest of the remaining races and post GP award ceremony. Also a small detail is that it was the last GP rival before unlocking Super Sonic.