Admittedly I'm not sure that this is a bug. It could very well be intended behavior. But it seems that, if you go to sleep with enough exp to level up, but also it's your first sleep since doing the "Romance" scene with Atugia after unlocking her, you do not actually level up. Maybe it's not technically sleeping and not technically a bug, but it feels like it would be a bug.
Browser is firefox, though I doubt it matters for this bug.
Also attaching a save file, though you can reach this point quickly from the start.
Browser is firefox, though I doubt it matters for this bug.