I know nothing about Music Brainz, but isn't it inevitable that something with humans involved will sometimes have errors? Is there such a thing as a restaurant that doesn't sometimes get a customer's order wrong, or a team of website moderators (paid or unpaid) who don't occasionally make the wrong judgement call, or a piece of software that occasionally had a bug that wasn't spotted in QA?

To go from being a "huge fan" to refusing to use it after a single mistake seems a very extreme reaction, unless that incident caused you to look into it more and discover that they're constantly making similar mistakes.

For something like restaurants there's often enough competing options to let a single mistake sour you to a place and choose alternatives in the future, but even then it a restaurant I was a huge fan of messed up a visit of mine I'd most likely give them another chance unless they literally didn't care care that something had gone wrong suggesting it's likely to keep happening.

edit: Also curious about what their rejection reason was that led you to describe it as "innocuous", seems an odd adjective to include without context.