It seems as though I should have run into this issue on previous Python versions, so unsure what may have changed in Python 3.13 to trigger this. However, the following code worked fine in Python 3.10 ...
In the "Execute Python" function, when JSON-like data is sent out, it is added to the "message" field. This leads it to LOOKING like it's valid JSON on the Javascript (frontend) side, but it is not ...