I have encountered a similar situation but it is with
[spoiler]Halsin[/spoiler]
I recruited him in ACT 2, backtracked to the mountain pass for exploration with him in a full party. After any fight in that map, he would cease to follow the party. If you have him lead the party, everything seems fine and other companions will follow. But if you make him a 'party member', he will just stay in place. This bug does not happen when he is engaged in the fight in the [spoiler]ACT 2 cursed land[/spoiler] and he will continue following after a fight.
I have also tried the strategies you mentioned:
-Reloading the game (IE closing the application and fully restarting)
-Saving and then loading the game again
-Splitting the party and rejoining it
-Pairing him up with a different companion and splitting the party that way
-Reloading a save before recruiting him and doing it over
-Having him jump into a chasm and reviving him with revivify
-Preforming several actions as him, such as jumping, attacking, moving objects, going in and out of stealth mode and then switching back to my primary character
-Attacking him myself
-teleporting/fast traveling to a different way point
-Dismissing him in the field and then recruiting him at camp again
-Dismissing him and then preforming a long rest
I haven't had wither revive him though but I doubt that makes a difference.
