admin-post is being blocked

I have a plugin that has a form with the action set to admin-post.php with the corresponding add_action defined as is the normal way to handle a form in a plugin. When this form is submitted, I get the “The admin of this site does not allow users to access the wp dashboard” error generated by Simple Membership. If I deactivate SM, it processes the form correctly. The odd thing is on my staging site, with all Simple Membership settings set identically, the same form processes fine with Simple Membership active. Needless to say, this is a head-scratcher. Any ideas what would cause Simple Membership to block this form submission? All plugins are up to date. Using Elementor for most pages. Astra theme. Disabling plugins and changing themes on my live site is not an option. That’s why I have a staging site. But the two sites are behaving differently in this one case!