Related to forum thread: https://www.telerik.com/forums/fiddler-no-longer-capturing-web-service-requests-in-net
We found that .NET Core projects use WinHTTP instead of WinINET. As Fiddler registers itself only as a proxy for WinINET, traffic from these projects do not show up in Fiddler. While a manual workaround exits, it would be good if there was at least an option to register Fiddler as a proxy for WinHTTP as well.
On a side note: We've seen that even node.js traffic does not show in Fiddler as well. We've not researched the cause as it wasn't critical to us at that time. But, it could also be due to the same reasons.