(oh, I'm on xbox 360 too, so no console stuff.)

I had an awesome dead thrall, forsaken ravager (level 34?) with legendary double
enchanted forsworn stuff, dual wielding dragonbone sword/axe and using dragonbone bow.

When we were with serana, on the quest to find her Ma, the dead thrall could not follow to the courtyard
for some reason. I zoned back in, she was just hanging there.

Later, after going to the soul cairn, "reanimate thrall" is no longer listed in my active effects (meaning the
spell is no longer "on." )

The dead thrall corpse was nowhere to be found, after leaving the soul cairn (even upon rezoning
into the previous area I saw her, just before the courtyard.)

Evidently She poofs?

I then accomplished too much of the quest, getting what I needed and going on to the elder scrolls, to
try to figure out if killing her there or something would make it so I can then reanimate her after getting back.

Its not that hard to find some forsworn again, waiting for the female dual-wielding ravager type I like,
but its irritating to have to enchant her gear again and give her new bone double enchanted axe/sword/bow.

In searching, I found nothing about this issue, but I guess if you have a dead thrall you like you should maybe save before entering the courtyard/kill the thrall and hope the corpse is still there upon getting back from the cairn?

There is no safe place to store a thrall simply safe corpses bodies that never disappear make good thralls as you can simply leave them in your house forever.
My advice is store the equipment but forget about the body.

I will also pointout that I've found both thralls and followers will dual wield forsworn weapons.

No, she is a forsworn ravager dual wielding the BONE weapons I gave her (sword and axe.)

I pointed that out because something I read, about the forsworn ravager as a good melee dead thrall (which it is,
actually quite amazing) was that you need to find one you see dual wielding so that it will dual wield as dead thrall?
Not sure if that is true.

Yeah, I guess I could have just got all of her equipment/weapons. Too late now. Guess I'll know next time.
I assumed that when I was all done with the thing, she'd still be waiting before the courtyard zone in.
I don't feel like going back to the save now.

At least with the forsworn, they supposedly will wear ANY forsworn gear when a dead thrall (while other
ones only wear the gear they came with.) So you could remove the legendary, double enchanted stuff
then put it on a new forsworn ravager later I guess. I'll have to try with the next set I make. I know
I already have another legendary set in my house, I'll have to enchant it up again for bow/one hand
then go find another one to thrall.

I'm skeptical about what you've said, thralls only ever wield their 'base' gear. With the exception of forsworn, they will wield any forsworn weapon, it doesn't have to be the one that they died with.

But unless the weapons are their base weapons, they won't equip them as a thrall. So I doubt they wield dragonbone weapons.

Thralls will wield any weapon but will only wear the armor they died in. Forsworn are an exception in that they'll wear any forsworn armor.

Thralls will wield any weapon or shield.

Forsworn will also wield any weapon.

Thralls will only not equip other armor, except for forsworn (as previous said.) Forsworn will equip ANY forsworn armor,
it doesn't have to be what they died in.

So a forsworn makes a pretty easy to replace thrall, if you know they are going to go MIA. Summon a junk thing, loot
the forsworn, replace all that stuff later into a new forsworn (even if its legendary stuff, enchanted, etc.)

It took a bit, but I did find another dual wielding ravager lady. I stuck some new forsworn 4 piece, legendary, double enchanted
armor on her and a new double enchanted dragon axe/sword combo. Then we went to town on some more stuff

What I found was, if you find a ravager who just uses a bow mostly and then a dagger then she won't dual wield. The
same is true of a ravager "caster." I got the bow one first, with the dagger she may also pull out. She mostly used the bow,
would not dual wield the sword and the axe but did pull out the sword once (when the dagger was replaced.) Then I found
another one already dual wielding, when attacking me. That one happily uses the dragonbone axe and sword, dual wielding,
while using the dragon bone bow from a distance.