In short, the 'src' python module of perfetto 'src.trace_processor' can't be found if there is a system python module of the same name. 

Only affects QtWebEngine 6.6+, doesn't affects QtWebEngine 6.5.

Please take a look at:

https://github.com/google/perfetto/issues/2579

https://bugs.gentoo.org/915615

This is already a two years old painful bug which gave us a lot of headaches.

Proposed fix:

Until upstream Google perfetto releases a fix, Qt should rename the module to avoid confusion with system package.

p.s.:
As you can see in the gentoo bug tracker, I had to wipe my computer because I was unable to apply the system upgrades due to qtwebengine 6.6+ would never compile.