Just a speculation, but I think when Frieren comes back to her own timeline, it will turn out that she spent XX amount of years in that dreamlike state and her disciples have probably moved on, married each other (or other people idk) and already had kids or something. From their perspective, she just disappeared suddenly and they couldn't find her everywhere. After years of spending time trying to find her, they finally gave up and moved on.
Hopefully that doesn't happen because I'd like her to witness her disciples' growth over time.