Was driving today from London to Birmingham. At certain points on my way, without any signs on the M1 changing, the car went from 70 to 50.

 

on the way home, twice the speed went from 70 to 100.

 

On the way there, the reduction was annoying, on the way back, it felt dangerous 

What year is your car? If it's an early one, it'll have an absolute tonne of updates outstanding, one of which will fix (or atleast massively improve) this issue 

2023

I’ve had this repeatedly on mine with travel assist.  Updates have improved matters, but not gotten rid of the bugs entirely.  I think some of this may be due to errors with the maps.  I note that there are points on the A14 where this happens consistently.  Other places, it seems random.