Cinnamon: When language switching uses alt-shift, alt-shift-tab doesn't work for cycling windows

• Cinnamon 3.8.8
• Linux Mint 19
• 64 bit
• xsession-errors.txt

Issue
I've set my language switching shortcut to be the familiar for me alt-shift. Since I did that though, I cannot use, or even set, the alt+shift+tab for backward cycling of the windows. The system acts as if I clicked alt+shift.

This used to work for me on Mint 18.

(First reported here: https://forums.linuxmint.com/viewtopic.php?t=272923 by someone else)

Steps to reproduce
Go to Keyboard->Layouts. Select "Options", and expand "Switching to another layout". Select "Alt+Shift" and click "Close". Try clicking "Alt+Shift+Tab" repeatedly. The window switcher will cycle forward, just as if I didn't click the Shift button at all.

Expected behaviour
The window switcher should cycle __backwards__.

Other information
Looking through .xsession-erros, I see the following error message:
Cinnamon warning: Cannot bind "switch-windows-backward" to <Shift>Tab: it needs a modifier such as Ctrl or Alt.