This looks like a regression of Plasma 5.8 BETA. When pressing Caps Lock in lock screen, the message "Caps Lock is on" shows up only for a moment and then disappears regardless of state of Caps Lock. After that in case Caps Lock is actually on -> when moving cursor or pressing some letters, message is displayed correctly. Testing on Arch LInux with testing+kde-unstable repos enabled: Plasma 5.7.95, KF 5.26. Reproducible: Always Steps to Reproduce: 1.Lock screen 2.Press Caps Lock Actual Results: Message "Caps Lock is on" is showed just for a moment. Expected Results: Message "Caps Lock is on" should be showed and stayed that way until Caps Lock is pressed once more.
Sorry, I've just tested it on "real" hw and this looks like VirtualBox-only issue.