I have never had a shop keeper un-assign themselves.
My shop keepers will all do business with me.
I have all 6 trading stands set up in Sanctuary and all upgraded to LVL 3. With Vault-Tec guy assigned to the Trading Emporium. They are all at their stands during normal trading hours, and they all barter. Sometimes, they still stand at their trading posts in the posture they use when they are open for business even when the stall is closed because it is outside trading hours. This results in them not having the Barter dialogue option, even though they appear to be manning the trading post. Wait until working hours, and they will barter.
My other settlements all have at least a couple trading posts at various levels, and they will all barter with me.
As someone else noted, the only oddity is with LVL 1 weapons stands. The settler assigned will usually stand somewhere nearby, instead of behind the stand where he belongs. However, he will still barter with me if it is during regular trading stand business hours.
Nevery had any problems with making sure there was level terrain. In fact, my trading posts at Abernathy farms all have at least 1 leg floating in the air because there is literally NO flat ground at Abernathy Farms.
edit: Vault-Tec guy is apparently the only so-called tier 4 merchant in the game that actually works. I have started threads in the past asking if anyone has actually gotten any of the other tier 4 merchants to actually man-up at a trading post. No-one has ever commented that they have actually gotten this to work with anyone besides Vault-Tec Guy. He sells a couple of specific legendary items, and does not follow the supposed tier 4 behavior of occasionally selling random legendary items.