The image I actually use is :
lsb_release -a
Release: 10.0
Codename: byzantium
uname -a
Linux pureos 5.9.0-1-amd64 #1 SMP Debian 5.9.1-1 (2020-10-17) x86_64 GNU/Linux
I think I downloaded the Build #6355 (11 janv. 2021 19:32:31)
I tried to launch qtqr with QT_QPA_PLATFORM=xcb (when default is bash) : it works !
it doesn’t explain the working behavior with zsh, but is enough of a solution/explanation for me ! thx 