I tend to do something similar - setting their attack range to high, but their chase range to the minimum - I’m not certain it makes a difference, but it seems to. That said - I think, from the way the OP describes some of these incidents, it may be a different issue. Several of us have experienced followers ‘wandering’ of their own accord (the bug report I posted on it has a particularly clear example that just happened to be caught on video) - but a lot of the discussion has taken place in a playstation thread, because people initially thought it was only happening there.
A possibly related issue that has come up since 2.5 (or possibly 2.4, I forget) is followers left on guard now sometimes falling through the mesh - you can often still ‘track’ them on the follower tab, but there is no sign of them - in admin ghost mode, I’ve been able to go and look for them, and sometimes found them standing on a bit of land beneath the land (not sure why it’s there, presumably a left over) - others have reported being able to find some by flying far further down into the void. But they can also fall ‘all the way’ and die from it. These two situations sound a lot like some of what the OP describes. Nearest thing I’ve found to a ‘solution’ is to try to only place them on guard on a player-built structure - it seems to help (though doesn’t stop the ‘wandering’). So far on testlive I’ve had no incidents, but I have very few followers (I think my horse turned himself around the other day, so the wandering may still be there) - I suspect that it is more likely to happen with larger numbers of followers and particularly if they are in relatively close proximity to each other. (Larger numbers is just logical - larger sample size = better data - whereas the proximity possibility is based on anecdotal observation - I had more incidents when they were left close together and fewer when they were more spread out.)