I think it's something like "/b/ is set to generate JPEG format thumbnails, /q/ is set to generate PNG format thumbnails, both boards set to generate <img> elements linking to the thumbnails using the .jpg file extension"

So when you change both boards to generate <img> elements with .png file extension, it corrects one while b0rking the other. You'll need to check the "$THUMB_SETTING" part of the config file to check they're both generating the same format

PS: I think the reason PNG was considered was because it possibly fixes (or hoped to fix) the transparent background color issue in thumbnails, at the cost of higher thumbnail file sizes