no window decoration on KDE5 / Qt5

I'm building a simple application with luce. It works as expected in KDE(4) on Slackware, but on Fedora with KDE5, it fails to get window decorations. This could be a Qt5 problem, not a luce problem (?)

Screenshots:
borderfull:
https://www.unixporn.pro/i/AHzfFDc.jpeg

borderless:
https://www.unixporn.pro/i/TTY7yRX.jpeg