In GP Premium, /woocommerce/functions/functions.php about line 808 begins a small section of code that applies when dynamic typography is in use. The problem is that even though this is pulling typography settings from the dynamic typo settings, it has a hardcoded units choice of ‘px’. The result is that when the button font-size is set in the customizer to be under 1, such as 0.8rem, then this code will output a font-size of 0px.
I propose that line 814 ought to be changed from
$css->add_property( 'font-size', absint( $data['fontSize'] ), false, 'px' );
to
$css->add_property( 'font-size', $data['fontSize'], false, $data['fontSizeUnit'] );
Lines 827 and 833 ought to be modified similarly.