The reason
ACF uses it’s own hooks to format the fields at priority 10. In other words ACF formats a post object field when the acf/format_value/type={$type} hook variant fires. This hook variant is fired before the other variants. This means that when the other variants fire ACF has already formatted the value on the type={$type} variant hook. So the priority for the other variants does not matter.
When in doubt always add a priority > 10 for all ACF filter and action hooks because ACF uses them too.
