Confirmed. This is indeed a bug in CoE4 v4.08.

It also lets them move while having the stationary tag.

I was just messing around with another mod and found that loners are still stuck as stupid. Does anyone know if this Is this intentional, or does it need corrected?

still not fixed as of version 4.25. which is a problem for my new mod. I have some powerful units that need the loner stacking disability to discrourage their overuse. but I cannot have these fixed in place units wandering away either. so either I will remove loner ability or force them to stay in place by making them commanders with sentry duty

I made bug report on this for v4.26.

In the meantime, I believe a simple workaround is to have your loner command followed by a clearmove (which clears all movement bitflags, no other properties). That seems to get rid of the problem, the unwanted movement bitflag 29.

After that, you can (again) set whatever movement bitflags you want, using those movement mod commands that don’t take arguments (i.e. excluding the commands trample and tramplexsize which add "effects" – i.e. integer pairs – to the monster definition rather than setting any bitflags there).