Release group C includes release group A and release group B for instance, is a relationships that is apparently not supposed to support credited as.
Current test server (password mb) acts same as current live:
1. Edit release group COMEBLACK / ACOUSTICA (test server)
2. Click Add relationship
3. Type anything in Credited as field, like PIPO for example
4. Select Release group type instead of default Artist
5. Select Included in relationship instead of default live performance of
6. Click Change direction
7. Search and select the COMEBLACK album
8. Click Done
Now the COMEBLACK album has been linked as PIPO, although the Credited as field has disappeared in step 4.
Hopefully, it's just a display bug in the popup relationship editor.
I submitted an edit and the credited as text is absolutely not kept in the end result.