I think the default for Roon is to hide albums it thinks are duplicates unless you change the setting (must be done separately in each device one is using to control Roon).
Did changing the setting on your iPhone work to show the albums like on your iPad?
PS - Looking at the Album Editor, Visibility in Roon settings, I notice beside the Unhide button it says this is to unhide an album you selected to be hidden. I interpret this to mean it only applies if you previously selected that album to be hidden, and doesn’t apply to “duplicate” albums Roon hides by default.