Nice find!
Seeing this, I suspect the behavior of the two javascript engines differs for this particular regex pattern. The botengine for Windows uses the ChakraCore javascript engine. In the alternate UI, it depends on which browser you use, so that could explain why you see it working in the alternate UI.
I opened the alternate UI in Chrome and Edge and loaded the sample you linked. I see the difference there too:
I am surprised it did not work in this Edge, I had expected that Microsoft switching to chromium would lead to using the same interpretation of regex.
We can also see the difference in this regex testing tool: regex101: build, test, and debug regex
