When I go over bumps you can hear the base and seat wobbling in the back! I’m not sure about other brands but if I remember rightly, the cybex cloud (which I have) is actually safer being fastened in with the seatbelt than on the isofix on all of the testing & ratings.
I think depending on the brand the base is meant to move slightly to prevent such a harsh impact if there was a crash! (Mine slides side to side)