Well Fiddler does chain proxies but it does that only once on startup. I.e. if there is a proxy set when Fiddler gets started Fiddler will replace that proxy with itself but then it will use that proxy for all outbound traffic.
If another program changes the proxy settings once Fiddler runs, though, Fiddler won't change them back.
In the xscaler/Charles case it looks like zScaler does the chaining and not Charles. zScaler seems to support specifically Charles, though.
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