Well, the title says it all. It is working correctly, and able to capture traffics and view them. But if the option to "Capture Traffic" is disabled (e.g. via F12 or File menu), Fiddler still captures traffic as usual, as if the option is enabled. The status bar correctly shows the capturing state.
Running Fiddler Beta v4.6.20171.26113 (Built: Monday, June 12, 2017) on Mono 5.2.0.215 64 bit on Ubuntu 16.04 AMD64.