Incorrect group when group selection is not mandatory

I skimmed through 15 pages of the support forum and couldn’t find anything similar to my issue. The plugin has been working well for a while now, I just made a change that no longer required the group selection to be mandatory and it’s not working properly anymore.

I have two groups created, but only one group showing on the form, and if a subscriber doesn’t want to be included in that group, they don’t have to select anything and it should automatically subscribe them into the Public group. However, if they DO check the box to be included in the group, the plugin still subscribes them into the Public group only, which is wrong.

Any help would be much appreciated. For now, I’m showing both my group and the Public group on the form and set the group selection back to mandatory, but ideally I should be able to show one group and it shouldn’t have to be mandatory.

Thanks in advance