Anyway, thaks for RIGHT link - link to information about xorg bug (in XCB):
https://gitlab.freedesktop.org/xorg/xserver/-/issues/257
https://github.com/sddm/sddm/issues/202#issuecomment-133628462
Strange, why it presents in Lubuntu > 19.10 install from LiveCD and DOESN’T present after installing “Lubuntu Desktop” with, for example, tasksel (even from Ubuntu netboot LiveCD [yes, it’s still exist for Ubuntu 20.04, but last modified in 2020-04 - still can install up-to-date Ubuntu and even Lubuntu])