Bug#870641:

[dinar qurbanov]

possible solutions/wokarounds:
1) https://bbs.archlinux.org/viewtopic.php?pid=1671675#p1671675
2) https://github.com/the-cavalry/light-locker/issues/108#issuecomment-502404552
(i have not tried these).
(i use ctrl+alt+f6 then ctrl+alt+f7 as a workaround for now. )