I downloaded the latest version on website for Fiddler Everywhere Beta (v0.10.2) and upon fiddling around with it I found this reproducible issue. I'm running macOS Catalina (v10.15.3).
When you quickly and repeatedly click on the toggle for live traffic capture, fast enough that you don't let the toggle complete its animation / switch to either state, more than a couple of times (perhaps 10 times or more), the UI keeps toggling when you do and will settle on one state when you stop clicking.
But after a few seconds, the toggle will starting switching again. Sometimes it takes more than a few seconds. It toggles a couple of times, and even if it finally stops itself at "on" state, the traffic is no longer captured.
A user might not try to deliberately break the app like this, but perhaps there's something async going on with the UI that makes the actions happen later and also break the capture part.
Not sure what kind of system details might be required, but here's some generic info:
Application version: v0.10.2
OS version: Catalina (v10.15.3)
Processor: 2.3GHz Intel i5 (8GB DDR3 RAM)