Cannot use REST Adapter - relative path must not contain a query string

Summary:

Hello, was there a new update or patch to REST Adapters in GEN3 that changed their behavior to no longer allow relative paths to contain a query string?

I have a REST Adapter failing to make a call that it has successfully made several dozen times before, including in a run yesterday.

This is very unusual behavior because this call has never failed to go through prior to today. I put the Relative URI in the Content section below so you can see how I'm forming it. I do not see any other way for me to make this call other than a concatenation, as the 'Principal Investigator' must be dynamic.