Why is one of my Input Sources intermittently not working?

I created a Language Keyboard (Input Source) with Ukelele. It worked fine on my BigSur. Now I moved the file to my new Macbook Pro Monterrey to

 /Library/Keyboard\ Layouts/HebrewYiddish.bundle  

Sometimes I can switch to it, sometimes I cannot. I can't figure out what conditions this.

When the problem is happening, selecting the keyboard in the taskbar [1] just sends me back to another Input Source. Likewise with the keyboard shortcut for switching Input Source (which pops up the new Monterrey keyboard-switching overlay GUI).

In Settings, the keyboard image shows the keys correctly.[3] Strangely, the icon of the problematic Input Source in this view is, when unselected, black, while the other two (functioning) ones are in gray.[3]

The problem occurs whether I have one or two healthy Input Sources in addition to this one.

How can I keep the Input Source consistently available?

[1]

[2]

[3]