Hi,
Normally the rule is like this:
The category type of an element is copied from its parent category type. However, if the parent category is the "root" category with the category type "root" (which is like that by default), then the category type is not changed.
So if the category type of the main manufacturer changes to "product" when you edit it, I suspect that the root category's type has been changed from "root" to "product" leading to the issue you have.
Please check that category's type with phpmyadmin and restore it and it should then work properly.