Unable to update 'Fixer' child theme

I currently have the 'Fixer' child theme installed, and I have recently updated it from 1.1.1 to 1.3. However, it is still showing in the Updates list on both WordPress and the WPMU DEV Hub Dashboard.
Any attempt to update this returns that 'There was an unknown error'.

It seems that I have two versions of 'Fixer' in the Upfront Builder:
- One is Active, and seems to be the one I am using. Checking the version of this (Upfront > Builder > Edit Theme Info) seems to be 1.3 which is fine
- The other is Inactive, marked as a CONFLICT and only on version 1.1.1 which seems to be causing the issue

I have attempted to delete the Inactive theme (Appearance > Themes > Inactive Fixer Theme > Delete) but am getting the response of 'Deletion failed: Could not fully remove the theme'.

Can you please advise?

Thank you in advance!