Redmine

"Category" is depends on project.
Category ids not same even if category names are same.
I think clearing category is expected behavior.

I guess this is a bug because Redmine behaves inconsistently here: when you edit multiple issues and change the project, categories with the same name will be preserved - see project setter for issues:

if category
    self.category = project.issue_categories.find_by_name(category.name)
end

When you edit a single issue and you change the project, the category should not be cleared but should be set to a category with the same name (if available).