The Fieldmanager_Datepicker JS isn't working in WordPress 5.6 on the Gutenberg edit post screen, but it does work fine in WordPress 5.5.2.

It also works fine in WordPress 5.6 for the classic editor.

Code to reproduce the issue:

add_action( 'fm_post', function () {
    $fm_date = new Fieldmanager_Datepicker( 'Date' );
    $fm_date->add_meta_box( 'Date', 'post' );
} );