I think something about this was talked in a thread about making kokonotsuba open source.. But I am not sure if it's an easy fix

It's possibly related to this post >>62271

I had to do the same fix0rz on my test instance as well

fixed
Interestingly, it was properly working on /b/ despite the bug

fixed for /q/*
still b0rk on /jp/ it seems

Then it breaks on /b/ too. Decided to revert after doing some tests
I can't say I really understand how it works... What works on /b/ and /jp/ contradicts with the one for /q/. My uneducated guess is that it's something to do with the old imported database from gurochan (also the reason /q/s numbers are so high).

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