I do know that it works/or does not work when mixing acf options pages with other types of admin menu items. I don’t know why you are seeing this issue when using a WP sub menu page as a sub page of an ACF options page.
This has a lot to do with the priority when adding the page or sub page. For example, due to priority of the ACF options page addition it is quite complicated to add a CPT as a sub page. It could be that there is a similar situation happening with mixing and acf options page with wp sub pages. See my explanation and fix for adding a CPT as a submenu item of an ACF options page here, it might help you. https://github.com/Hube2/acf-filters-and-functions/blob/master/acf-options-page-w-cpt-children.php
