I have been tasked recently with finding a way to validate JWT that can come from multiple different identity providers, in the same API. This tends to happen when companies buy each other and try to merge their products and user base. So, first, let's be clear about one thing: wrangling multiple IdPs is the path of pain. You would think that it would avoid painful migrations and get you up and running quickly, but you will pay a huge complexity tax when juggling users and roles from multiple systems interacting in your application.
Validating the JWT, though, is manageable, but I've not seen good advice on how to verify them when they were produced by multiple identity providers. So, let's find a good process for that.