Try this:

for (k in AllKnobs) k.set("showValuePopup", "No");

No, that determines whether the callback is executed on mouse down or mouse up. Regardless of a script being attached to it.

By default, a button changes its value/state and the callback is triggered when you release the mouse button.
If you activate this, the value/state of the button changes immediately when you click the mouse button. This feels more snappy.

This has already been discussed somewhere here.
The “Extract Metadata for Mapping” button is not in sync with the internal variable. That means it may appear to be on, but it’s actually off. A simple workaround is to toggle the button off and on again.

@Christoph-Hart , I think this can be easily fixed in FileImportDialog on line 39:

metadataButton->setToggleState(true, sendNotification);

Click the 'Extrac Metadata of mapping' button off and then on again. That should do the trick.