Hi @ThePowerFox ,
Have you tried the new Sharing UI dialog within PowerApps to share your Model Driven app?
Based on the issue that you mentioned, I have made a test on my side, I suppose that you are trying the new Sharing UI dialog within PowerApps to share your Model Driven app as below:

is it right?
Actually, it is an known issue with the new sharing UI dialog -- if you assigned a Security Role to your Model-Driven app, and click "Share" button, the assigned Security Role could not be unchecked again within the new sharing UI dialog. These assigned Security Roles would be grey and could not be edited.
As an fixed solution, you could consider go to your Dynamics 365 CRM instance (PowerApps Environment, where your current Model-Driven app hosted in) through the following URL:
https://orgxxxxxxxx.crmx.dynamics.com/
then go to "Settings"-> "My Apps":

then find your Model-Driven app there, and click "Menu Options"-> "MANAGE ROLES" option:

then you could edit these assigned Security Roles for your Model-Driven app:

Best regards,