Reading about about these instances and going by my own experience this seems to be linked, at least partially, to server crashes possibly resulting in decay timers being reset.
Say for instance a person logs in on Monday morning to refresh timers but the server crashes later that day before the normal server restart. My guess is the timer refresh is lost, at least for the thralls, so it’s as if the person never logged in at all. Then if they wait too long to log in again the thralls are lost.
There’s also an issue where changing accounts can cause thralls to be instantly deleted but that so far has only affected newly placed thralls for me.
All I can say for sure is that refreshing every three to four days instead of once per week or ten days has kept me from losing any thralls to decay so far. Even on the same servers where other people have lost them.