Updating substitution variables via REST API broke with 23.03 update

Summary:

In performing regression testing, I am having trouble using the REST API to update substitution variables. The rule results in a null pointer exception.

Content (required):

I am running a rule in Planning that updates substitution variables in FCC. The code currently works in production but does not work in test on the 23.03 update. We use defined Connections (Tools -> Connections) for this. See below for the connection setup and code. I have confirmed that it is pulling the correct application name, api version, etc. and that the credentials defined in the Connection Details are correct. This still works in production but may not work once 23.03 is applied to production.