Not sure if this counts fully as 'distributed' here, but we (Authentik Security) help many companies self-host authentik multi-region or in (private cloud + on-prem) to allow for quick IAM failover and more reliability than IAMaaS.
There's also "identity orchestration" tools like Strata that let you use multiple IdPs in multiple clouds, but then your new weakest link is the orchestration platform.
Disclosure: I work for FusionAuth, a competitor of Authentik.
Curious. Is your solution active-active or active-passive? We've implemented multi-region active-passive CIAM/IAM in our hosted solution[0]. We've found that meets needs of many of our clients.
I'm only aware of one CIAM solution that seems to have active-active: Ory. And even then I think they shard the user data[1].
There's also "identity orchestration" tools like Strata that let you use multiple IdPs in multiple clouds, but then your new weakest link is the orchestration platform.