Yep, thinking it’s specific to my environment, which is nginx reverse proxy (VPS w/public IP) in front of Authentik (in my homelab) in front of Vikunja (also in the homelab). I’ll get it puzzled out, or just continue as-is.
I had the same issue of not being able to add Vikunja to my home screen on iOS, it was just a Bookmark.
Turns out my Pangolin setup was at fault. After disabling Pangolins SSO, it worked as intended.
You can re-enable SSO protection after adding it to your home screen, but i have not tested how it handles reauthenticating with Pangolin.