I was using a staff at the time because mostly using Spark, so that makes sense why they couldn't be used at the time, that solves the mystery.
Yes, I met her in her room and I click Train instead of Heal, she then disappeared. She's not in my party (currently have 4 other members, but I believe at the time when I click on Train, I only had 3 other members, probably doesn't matter). She not following or in my party, so that's why I believe this is a bug. I have already fought monsters on the west/east side many times and already killed the boss at floor 3 right now and did the additional quest that was unlocked because of it. She's still not in her room and I don't know where she disappeared to. I've progressed a lot since that point, if it's a bug, I really hope there can be a fix and not just bugged forever at this point (Mellissa being gone forever).