It's been a long time.
I found out that some armor stands despawn like hostile mobs.
But some armor stands never despawn.
I tried to keep them by adding PersistenceRequired:1
but it doesn't work at all.

I do see some similar situations in Quick Search,
but it's far from 1.10 and isn't fixed.
So just a quick report to see if the developers
still know this issue.