Yes its same on my server they despawn so quick, if you get fix say that and if I get I say it to.

At the moment I found that only the desert titan works but you have to spawn it while you are there so it wont despawn, as for the other titans even if you spawn in the moment they will eventually go back to the center and regain full health so that's a no for those, I assume the problem is the way they wokr in the vanilla game