Did you ever get this figured out?
I'm experiencing the same issue with my GLC43. I'm taking it in later this week to hopefully replicate the problem with a tech in the car. It's common enough that it happens every day but not every time. Hopefully I can replicate it for the tech.
This seems to be a reported transmission issue/problem on both the GLC43 and the C43.
The jolting/bucking should not be confused with a hard shift that is common in Sport+ modes on most cars but rather a rough, harsh, bouncy feel as the car jolts and shudders forward (picture mechanical bull). In my case the car refuses to regain composure unless I switch it back to comfort mode. I've also managed to do it once in Sport, albeit, sport+ is where this issue is most common.
Real bummer... hopefully it's something that can be patched but at this point I'm not holding my breath.