Ideas
Thank you for reporting this! The issue seems to be that the final bearing can be picked up before the grind registers, thus ending the challenge. What I'd recommend in the meantime is grinding the bus on the first lap instead of the second/last.
The timing of bearing pickups seems to have changed with S3. I noticed this during the Skate Paddy's Day event, one of the line challenges at the Market Mile park had the same issue where it wouldn't count the grind on the final rainbow rail because you picked up the bearing before it could register. I don't recall this ever being an issue before S3 because it seemed like there was always a grace period after the final bearing was collected, but it's possible the challenges themselves were simply designed differently to ensure this didn't happen and that technique was forgotten with the implementation of speedlines.