There's been a long standing upstream issue to try to deal with a rather pernicious problem we have: that "Erase Disk" is not an option if the target disk has any partition mounted, most notably swaps, which are automounted in Ubuntu. There's been discussion of having a warning or a notification in Calamares, but so far, no movement. We have information about this in the manual but that hasn't helped entirely with the confusion.
My suggestion is to create a shellprocess to deal with this. I'm thinking it could tell the user that they have partitions automatically mounted that could keep them from using the "Erase Disk" option. It would recommend that the user unmount these partitions before continuing and give them a simple yes/no. We should give enough information that it identifies the nature of these partitions but not so much that it's intimidating to Joe User. Advanced users should not be a consideration; they can always open a shell or the partition manager or whatever.
If Cala does decide to fix this, we can easily drop the change, but this will have significantly improve the user experience.