JD Page Builder is no different in this regard from J! articles, categories or similar. Means it's according to J! conventions, behaves normal and as can be expected.
It depends on you and your configuration, so it takes you to understand Joomla permission rules and applying them:
In such instances, child groups can only do the same or less than their parent. Means, a child can inherit from its parent or be denied, but can not be allowed if the parent is denied.
By default, "Manager" is set to "Inherited" = allowed, so I guess you've changed most perms for "Manager" to "Denied". So, now its child "Language Minotaur" is denied as well: it cannot do more than the parent, so it cannot be allowed.
Here you will experience the same behaviour with "Administrator", who elsewhere can do more than "Manager", but not in these instances.
So, two options for a solution:
1. Either give "Manager" their perms back and set it back to "inherited". Then "Language Minotaur" can be allowed as well by setting it to either inherited or allowed.
2. Or configure the user group "Language Minotaur" to be a child of "Public" (instead of "Manager"), then set its basic perms under System -> Global Configuration -> Permissions tab (e.g. identical to "Manager"), and finally set its permissions per backend instance as you need them. (E.g., then "Language Minotaur" could have greater perms than "Manager" for JD Builder categories.)
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