When telling Marv to follow, and it seems to happen most when passing from the station to ship (but not always, sometimes immediately after recruiting), Marv's profile picture on the right hand bar will start to blink rapidly (maybe 2-4x/second?). Marv will continue to follow you, but cannot be selected/given order/etc.

It's almost like Marv is getting added/removed from the roster repeatedly/rapidly. Sometimes this will reset when moving through areas with the roster set to follow (IE: airlock > station > resolved), other times it will persist, and eventually Marv seems to disappear from the Roster, yet still exists on your ship/etc.

If you die on ship, you will not be provided the option to replace Marv, you will be re-spawned at home base, but then when you go to retrieve the ship, it will be missing, and Marv will appear piloting it (green ship color of same sort), even if you have already re-recruited Marv.

Playing with Harsh World mod, might be related (to the last point), the same behaviour occurs in vanilla (for profile blinking, Marv bugging out, etc.).