When I press the shortcut for task switch for the current application, the task switcher does not work correctly (independently from the effect selected): it starts the chooser and I can see all the windows of the current application but then pressing the shortcut again does not allow me to switch to another window of the available ones (like you usually do with Alt+tab that you press over and over til you reach the desired window).

Is it a known issue?

I'm on KDE Neon by the way.

I'm seeing the same behaviour after the upgrade to 5.9 (and not fixed in 5.9.1 on OpenSUSE Tumbleweed). However, cover switch and flip switch seem to work ok, so that might be a workaround in the meanwhile.
I filled the following bug regarding this issue: https://bugs.kde.org/show_bug.cgi?id=376399

In my case (having Intel graphics and following Martin instructions in the bug report) this issue has been fixed by switching from EGL to GLX by removing the line in "kwinrc" (the hability to make this change from UI dialogs where removed in previous versions). That is, remove the line containing "egl" in "$HOME/.config/kwinrc" and restart the session.

After a recent upgrade (in KDE Neon), this now works correctly...

Plasma 5.9.1
Frameworks 5.30.0
Qt 5.7.1