word completions: ctrl-tab won't cycle / alternative words does not work

In Writer, keyboard shortcuts like ctrl-s, ctrl-z, ctrl-alt-up, etc works, but ctrl-tab to cycle through alternatives to suggestions for word completion doesn’t - when I hit ctrl-tab, Writer reacts as if I only hit tab and inserts a tab stop.

I notice that in Tools > Customize > Keyboard the shortcut ctrl-tab is empty.

How to enable this shortcut?

I’m on Kubuntu (Plasma desktop) 20.04 with Ubuntu Studio packages installed.

Build ID: 1:6.4.6-0ubuntu0.20.04.1
CPU threads: 8; OS: Linux 5.4; UI render: default; VCL: kf5;

It works for me in LO in Ubuntu 20.04.2 LTS. No function is assigned to Ctrl+Tab. Are you sure that you press Ctrl and Tab at the same time?

Installing LibreOffice 7.1.1 from the fresh ppa https://launchpad.net/~libreoffice/+archive/ubuntu/ppa does not change anything. The problem persists, and yes: I am sure I’m pressing CTRL and TAB buttons at the same time (as when using other, working shortcuts involving the CTRL button).