I'll check these issues.
@Metrux Are you using the newest version? Going to the bathhouse in Aldlyn might solve the problem, as your group gets removed when you enter and added again when you leave.
@Dark Kaplah Unless you turned it off, the game makes auto-saves every 2,5 mins alternating between save slot 19 and 20. Check if maybe one of these saved shortly before you had the issue.
Yes, I am using the newest version, I did use the bath house to get the rest of my team back. But Sandra didn't come with them. Right now I can't progress because I need her for some quests, and I don't want to advance the main storyline until I have her. I did try using the bath house again, afterwards, but she still won't come back.