I have now experienced this bug twice in consecutive MeetUps in France, both on Petit Boucle route, when there are only two riders and we are in Keep Together mode that we get capped at 16km/h no matter how hard we pedal. On both occasions someone late joined the group, no idea if that is affecting this bug or not. The first time we were a group of four and once two people left then two of us were hard capped at 16km/h even descending Petit Climb. Once my friend left then I sped up again to a normal pace. The second time only two of us started the MeetUp and it affected us from the start. We quit, created a MeetUp in Watopia and the problem went away. I can’t imagine it’s specific to Petit Boucle, maybe the fact one of us late joined each time is in play as that code is new. It only occurs in France which is also new code. You should be able to easily reproduce this and remedy. Feels like 16km/h has been hard coded or misconfigured somewhere. On a flat road with two of us pushing 4.0+ we shouldn’t be capped at 16km/h
My impression is that Meetups with Keep Together and only two participants quite often (no matter the route/map) have problems with speed - either too low, as you describe (as if there’s an invisible third rider way back keeping the speed down), or too fast (like you’ve got a third super-fast rider off the front towing you along). This is when the two riders are right together.
I’ve experienced both the “pedalling through treacle” effect and the “flying” effect, with two riders who are right next to each other. It doesn’t happen every time, but it is quite common. Sometimes the effect goes away for a while when the terrain changes, but it comes and goes.
It’s clear to see that the Keep Together is going haywire because you can push out lots of power and still be crawling along, or you can find yourself flying uphill at a speed that you know is unfeasible.