Forgot to mention the exact environment in my first post:
Master PC: Windows 10, running Veyon Master/Configurator 4.9.x
Linux Client PCs: ALT Linux–based school distribution (MOS/ROSA, used in Russian schools), also running Veyon 4.9.x
User accounts on Linux: user (regular), also system accounts like mos-admin, mos-teacher, mos-student
Service manager: systemd
Network: same subnet for all PCs, firewalls disabled