I have tried this in my lab and it is working fine for me without any errors.
I created 16 groups and assigned these groups to one of the authentication method policy.
I am able to delete any group from that policy or I am also able to make any changes in the same policy.
If you are still facing issues, we can connect offline to fix this issue.
Please send us an email on azcommunity [at] microsoft [dot] com with Sub - Attn: Sandeg and following details in the email body: Link to this thread/post We can connect offline and discuss further on this.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.