I have recently found an issue with skill trees of my fully upgraded characters. It shows that a tree has no upgrades at all on the general character view (where you set your blockbusters). When I open the tree itself, skills are actually upgraded, but somehow locked behind again. When I try to unlock a section of the tree, I'm loosing a key, so basically I have to reopen my upgrades. Though, if I have no keys at all, I can open sections for free. I've attached some screenshots of my most upgraded characters in hope of making it clearer.