Same issue as this: VNC: pressing Shift + Tab = Tab only
I can write "A" by pressing shift+a, but shift+tab won't work. Tab be triggered without the shift key. This has been an issue since at least 16 I think when I started using the gnome variant of Ubuntu. Right now I am using PopOS 18.04 and with both tigerVNC and Real VNC Viewer (on Windows 10) I have the same issue connecting to the Vino VNC server. Without the VNC connection shift+key works as expected on both Linux and Windows.
I also tried with tiger VNC viewer on the Linux subsystem. Same result.