Some dead keys not working correctly (layout: cz_qwerty)

Subset of dead keys is wrong for czech keyboard.

They keys starting with dead key ' (áé...) are correct while the dead keys starting with ˇ (ďťž...) are all messed up (one gets ﻾ and so on).

I use standard debian stretch install and use the following keyboard:
setxkbmap -model pc105 -layout "us,cz_qwerty" -option "grp:alt_shift_toggle"
 
Non-qt based apps are getting it right (firefox,libreoffice,terminal) but any qt-based gui is broken. I also tried already locally compiled 5.10 release with the same result.