Monspace font as squares

Hello !
I've been having the strangest bug with KDev5 (all version up to 5.3 beta1)
: The monospace font (the default one) is only showing squares (see
attached file).
This seems similar to
https://mail.kde.org/pipermail/kdevelop/2018-August/019690.html.
So idea what happened on my ubuntu 16.04. I tried regenerating the cache
with sudo fc-cache -r -v -f, copying /etc/fonts/ from a working computer,
copying /usr/share/font* from a working computer, regenerating, rebooting.
Nothing.

In the end I can change the font and I can see the code, build the build
windows is staying at Monospace font even If I change the editor + KConsole
to something else.

I see this Monospace font was an issue at some point :
https://cgit.kde.org/kdevplatform.git/commit/?id=12f5de1c9706b7b5b49528b57acfe4a350110839

Thanks for your help !

Antoine that really wants to use clang-tidy