I select any icon other than the default red icon, Bear. Then I select the default red icon, the selection is reverted to the previous selected icon after restarting the app.
That’s how selectable icons work on macOS I’m afraid. When you quit the app, it reverts to whatever the default is, or the one you’ve added via the Get Info menu.
Thanks @R2000. I think my wording was confusing. I meant to say I wanted to select the default icon, but after restarting the app, the icon reverts back to the last selected custom icon (the one I used before selecting the default icon).
However a reinstallation fixed this. Might have been an edge case. Leaving this here in case it is helpful for the dev.