When entering all info in a new ticket (Template), and then change the Category, it will clear out the fields that had already been filled in.
It will remove the team and tech by design as they may not have access to the category you are choosing. When making a category change you will be required to assign to the team and or tech also. The other fields will hold.
I had to check it again this morning, I understand what you are saying, but the custom fields also clear out. We only have 2 at this point so it's not a huge deal.