Has Istiana disappeared for anyone else?
Managed to sort of work out what's happening (will raise a bug) - pretty weird, and fairly significant defect.
Game started on 1.5.7 beta.
When loading a save made on 1.5.8 beta, any 'unseen' NPC's (Istiana/Arzagos, and many potential companions; NOT nobles; not sure about town/village notables) are broken. The bug happens on
loading the save; the characters are fine when saving, fine continuing the game without reloading, but gone on reloading.
Arzagos and Istiana remain in the encylopedia but location changes to "Never seen before" and they're not at their normal location (=quest line broken).
For those who don't care about the main quest:
Potential companions you've never seen at all (i.e. not companions you've hired, and not possible companions you've 'seen' in a town but not hired) are deleted entirely - they no longer exist in the encylopedia. (Spotted this because loading various saves and typing Istiana's name to check her status, a possible companion 'Istiona the Healer' stops appearing as you type in the same circumstances as Istiana goes missing).
**Actually, I'm pretty sure I had seen Istiona the Healer, so not sure what's making some disappear and some not; I'm 99% sure though that all the wanderers that did survive the 'load cull' were ones I had seen/recognised the names.
As a quick test, just started a new 1.5.8 beta game, skipped the tutorial, checked there's a page and a half of wanderers, saved/reloaded, and still a page and a half of wanderers -
so this appears to be quite a significant bug breaking 1.5.7 > 1.5.8 save games, but not present in 1.5.8 fresh games. Unless it only occurs at some later point connected to the main quest, but I'm not chasing 10 nobles and fast-forwarding a couple of hundred days right now to find out...