Possible Bug Report: How do I figure out why attempting to edit my OpenSearch Access Policy results in a 409 error?

Steps to reproduce: -- Navigate to OpenSearch Service in the AWS Console

-- Select an existing Domain

-- Select the "Security Configuration" tab

-- Click "Edit"

-- On the next page, make literally any change at all to the policy and click "Save Changes"

I consistently get a 409 error with the message "Error setting policy: <<JSON OF POLICY>>". There are no additional helpful messages or indications what's going wrong. I have full Administrator permissions to everything in AWS.

Any suggestions? What's going on here?