Schema is not configured for mutations

I am attempting to track down an issue with a production graphql-ruby app:

• I have a graphql app with two schemas. Both are configured with mutations.
• One particular mutation, very rarely gets this error and we have a front end error catcher that notifies us Schema is not configured for mutations: {"response":{"errors":[{"message":"Schema is not configured for mutations","locations":[{"line":1,"column":1}],"path":["mutation myMutation...
• We have over 60 mutations and only one mutation query ever seems to trigger this error
• Nothing looks interesting or different about this particular mutation query. In fact, I am able to run this query without failure any number of times, exactly the same way (same query, same parameters)

I did read through https://github.com/rmosolgo/graphql-ruby/issues/2359 but this issue is so old, I'm not sure if any of it is still relevant. What can I do to get to the bottom of this?

Start by comparing the configuration of both schemas and how the failing mutation request selects a schema. Review issue #2359 for related behavior and investigate the intermittent request path. Done means identifying the cause of the schema mismatch or documenting a reproducible diagnostic path and concrete fix.