Turning off "Automatically rearrange Spaces based on most recent use" keeps the spaces in the order I left them. That's nice. Three finger swipe between spaces when not using the preview seems to work.
However, swiping beetween the previews, it sometimes jumps to random places in the order - which is not nice.
Possibly a bug, but I might as well just write this as a letter to Santa because it's got more chance of being read than a feeback.
The setting is "Automatically rearrange Spaces based on most recent use" which explains why the behaviour felt so intermittent.