Hi Clive, I know it's been a while since you posted this, but I'm wondering if you might be able to help me. I'm having this same problem with my custom favicon not working. Unfortunately, I'm not fluent in code. I'm wondering if what you're mentioning above should be found in the Additional HTML settings under the Appearance menu (screenshot below). Should there be code in there? As you can see it's blank right now.
I thought I could change the favicon by just going to the Header settings under the Adaptable theme, and dragging and dropping it in. But now, I've lost the original Moodle favicon and just have the plain grey globe favicon.
If you have any insight, it would be much appreciated. Moodle 3.7.2. Adaptable 2.1.1. Thanks!