Where the hell are they listed in the UI after you've made one? Whenever I make one I can't find the place in the UI when I come back to gog after I've closed the browser.

I remember quite some time ago that there were complaints there was no way to easily find GOG mixes (even ones you made yourself). I don't think the situation has improved and I believe the 'easiest' way is still via a Google search.

You have to remember to save the URL yourself.
Or stick them in thread like this https://www.gog.com/mix/non_rpgs_with_unique_usps so if you lose it you can find them easier.

Raised an issue with broken GOGmix UI (headers went missing not too long ago), the support team said that it's pretty much an unsupported feature now. Shame.

What I do is visiting the store page of a gsme I remember that is part of the list, and I look for the GOG mix in the column on the right. At least that gets the job done.

Ouch, that is sad. And I finally got myself to make one just recently. :(