I logged on this morning and my templar had no items equipped. They weren’t in my stash or inventory, just gone. I had just put Unity and the invincibility relic on him last night and they were gone along with sword, shield, and other jewelry.
Wish I could get them back but I’m sure that won’t happen. Just wanted to mention it hoping you could figure it out and keep it from happening again.
I have read on the old forum that this can happen when someone joins your game. When it dismisses the follower it has a chance of deleting all of their items. It just happened to me and I found your post when searching for answers. I have not tried this but I read that the workaround is to manually dismiss your follower before opening your game publicly or inviting friends to join. It looks like this bug has been around since release on the Xbox.
It has literally been 8 years since the first time this bug was reported. But don’t worry, they’re “working tirelessly” to fix it. 
This just happened to me again. All ancient legendary items, some augmented. I’m fed up. This is the only console game I can play without making me physically ill but there’s no way I’m buying a new console and Diablo IV when they obviously don’t give a damn.
This just happened to me today and I am so angry, frustrated, and disappointed. Everything that was on him disappeared. My boyfriend and I had been playing and then while I was soloing, I heard the template say, “I have returned” like he does when he’s died, and revived. Except, I had the “follower cannot die” token on him and it was Ancient. He had almost all ancient gear and at least 2 low level augmented gems with life or reduced damage afixes. Plus, I was working very hard and super excited to behaving my best season ever and to be having fun playing Diablo 3 again this season. No, I am far from an elite or pro player, and definitely a casual player, but that doesn’t matter.
The fact they have known about this bug since inception, and done nothing to fix it, is really upsetting!.
Yup. I’m rank 161 DH currently and have been loving season 22. But the game just deleted my primal follower item and I’m beyond angry. A primal follower can not die skeleton key. Gone. Forever. Been playing for many seasons and never got one, just for that to happen. He also had max roll windforce, and a max roll ancient occulus. I’m pretty done with Diablo after that one. Can I not play any game in 2021 without it having a game ruining bug?
Following patch 2.7.0 install, I equipped a few Primal items in some of the new slots. Started a new game and ALL the followers equipped items are GONE. Not in my stash, inventory or ANYWHERE.
Doesn’t bode well for season 23, if peeps are going to lose all their good items.
This just literally happened to me also just randomly popped on and everything on them is gone can’t even push the greater Rifts now, what’s the point of failed to fully equip your followers and season 23 if it’s just going to be deleted
This bug has been around forever but is almost 100% avoidable when you’re aware of it. The main instance that causes the bug to happen is when your follower auto dismisses after other people join your game lobby. To avoid this, go to the “party” tab and manually dismiss your follower. If it doesn’t auto dismiss from you walking around after they join then the gear shouldn’t disappear.
Seems they should prioritise this with follower gear increased in relevance.
Notwithstanding there’s a work around for those that know it, but reach of that will be hit and miss.
It’s pretty gimp where a gear centric game can lose a bunch of gear.