A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
I have had exactly the same issue for a couple of weeks.
The only workaround I have found to work for me is:
- Remove the site from its association with any HUB site
- From within the site, browse to Site Contents > Site Assets
- Replace the __siteIcon__.png file with the icon that you want (note, there are 2 "_" before and after!)
- Clear cache in your browser and refresh the site
- Hopefully the logo will have changed
- Re-link the site to a HUB site.
I'm not too sure if the linking to the HUB site is an issue or not, it's just the way that I did it which seemed to work, but perhaps try it without first.