For those of us using Graphene, the above GitHub link references back to this article (Niantic's Pikmin Bloom 110.1 is partially softlocked on Graphene OS · Issue #4663 · GrapheneOS/os-issue-tracker · GitHub), stating that the issue may be the Play Integrity API checks. Graphene recommends that app developers use Android’s hardware attestation API instead of the Google Play version as a way to maintain security while still remaining compatible with more devices (see Attestation compatibility guide | Articles | GrapheneOS). I don’t know if everyone having this problem is running GrapheneOS or if it is affecting all OS equally but this may be a good place to start.
Niantic PLEASE fix this ASAP because it’s been almost a month since I’ve been able to play and it is really frustrating.