I don't know if anyone else ran into this bug but after I lost all companions and went through the bathhouse to get them back sandra never came back. I've gone back in the bathhouse many more times and it's stayed the same. Oh and it's on android, sorry didn't post that first
Are you sure you had Sandra in that save? The system saves all party members you had and adds them back to your party when you leave areas like the bathhouse.
The only thing I can imagine is that you had an event in which it wasn't saved that Sandra was in your party. Can you remember what event separated your party and caused the issue?