Earlier quoted context omitted.
that's a setting you can turn off. settings -> desktops and spaces -> reorder spaces
Does it work nowadays? Back in High Sierra days whenever I tried turning this off it did absolutely nothing and still reordered my shit.
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.