When adding the forum as an app on my phone via Firefox (PWA) I don’t see a logo. Likely the branding options in the Discourse admin panel are not completely configured. You can also setup a manifest for PWAs. Chrome has proper PWA support, in that browser you then see some extra info when the manifest is configured, really as if it’s an app.
At the moment the forum looks like this on my phone:
An example of a fully configured branding setup:
Do you know which icon it is in the settings? I suspect that we just have a fully green icon on a fully green background which makes it invisible
I was correct, and the implementation of this is wildly different between browsers. I created an icon that works, it doesn’t work the best in either browser, I tried, but it’s serviceable in both
It looks like if you set the “large icon”, other icons such as “manifest icon” will be set automatically if not set.
Maybe you use a PNG picture with transparency? Then I can imagine it merges with the background.
It looks fixed now! 
