the "Delete All Markers" script command does not work when only one layer is selected

Thanks for the additional details. It's on the backlog to investigate. 

The bug also reproduces on camera layers, light layers, and text layers. I am not certain about all types of layers. So it looks like a recurring issue that varies depending on the layer instance.

Update : the bug is real on shape layers! Give a try : Create a shape layer, right click, click on > Markers > Add Marker.

Then, right click, click on > Markers > Delete all Markers.
You'll see it's impossible to execute.

Very weird.

24.1 is currently rolling out to users around the world. Should be available to everyone in about 14 hours from now, so hopefully you'll see the update then. 

I restarted After Effects, now it works. Weird, right ?

My After Effects version is 24.0.3. Why don't I have 24.1 ?