There could be several reasons but it could be that you have continued a very old save (prior to v0.7.2).
You don't get experience ever or it just says 0 but then gives you exp? It could be a text error.
On both hard and normal so far I haven't found anything weird related to the girls' exp.
Join Discord, talk to me via DM and share your save with me so I can find out if there are any weird bugs. It's easier to track for me.
EDIT: In case of using an old save you have to start a new game, it's all detailed in the changelog.