BUG: Related List Buttons with Client Script action now erroring

There appears to have been a bug introduced over the last few days with Related List buttons that invoke a Client Script action.

Button configuration:

Configured Client Script:

Results:

The default loader is presented at the top of the page, and an error appears in the browser console.

The actual Client Script is not invoked.

Could we get an acknowledgement of this bug and an ETA on a fix as soon as possible please.

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