If you are using a template built base on Helix Template Framework from JoomShaper, you might get some issues when using Membership Pro on frontend of Joomla 4 websites. Below are details and solution:
It causes by a bug from the template overriding Joomla User Form Field. The solution is removing the override at PATH_TO_TEMPLATE/html/layouts/joomla/form/field/user.php
It causes by a bug from the template overriding Joomla Calendar Form Field. The solution is removing the override at PATH_TO_TEMPLATE/html/layouts/joomla/form/field/calendar.php