[icon-button]: using onIcon/offIcon not shown in button

[BePo65]

Describe the bug
If i integrate an icon-button with "icon toggle" into my application (following the example in the documentation) the background of the button is shown, but no icon (no matter if button is on or off).
If I switch to the "Using [on] property" example (with explicit mdc-icon tags) everything is ok.

To Reproduce
Steps to reproduce the behavior:
I made a stackblitz (https://stackblitz.com/edit/angular-mdc-agnan-icon-button) based on the template from @RikEskes. But it seems that stackblitz fights with angular 9.

As the demo in stackblitz does not show the effect in the "live window" I have includes 2 screenshots - one of the resulting display and one of the used html: