Upon further investigation online, I discovered this was a feature put in by Nintendo such that the brightness was permanently too high for First-party games when using an unofficial AV cable.This is fixed by looking for an official AV cable (at time of writing, not available from anyone on Amazon, but fairly common on eBay for between 4 and 6 …
https://www.amazon.com/Gamecube-Console-Indigo/dp/B00005YXRM