What do you mean by "codename specifically"?

I mean the Codename field from the fields dropdown.

Noted. I'm unable to reproduce at the moment. What were the exact settings you used on the field (name, description, field options, default value, order, defines sort order, etc...)?

I set the codename as RANDOM so it's auto-generated by Composr but beyond that there were no other changes.
https://compo.sr/docs/tut-fields.htm#title__9

I did as you indicated on my test v10 install... custom Gallery field of type codename with RANDOM default value. But I'm not getting any errors / it seems to be working as expected.

Did you explicitly name the field "_gallery"? Maybe this is another bug that was fixed in master but not yet released.

I didn't name the field "_gallery", I will try to reproduce the error once I get some time and will reopen if I do, but as you cannot reproduce it from what I said then I imagine it was "user error" somehow.