Not sure if this is a known issue, my admittedly superficial search did not come up with results, so..
What you were expecting to happen, and what actually happened?
I expected Empowered Beasts to start with full mana, instead if Forest Guardian is placed after an Empowered beast, its Lord of Beasts trait overwrites the Empowered trait of the beast.
What are the steps to make it happen again?
Simply use an Empowered beast like Spirit Fox and place it infront of a fully traited Forest Guardian. In the pre-battle trait resolve sequence Spirit Fox will first get Empowered with full mana and then get 'drained' half of its mana by the Lord of the Beasts trait.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!
No, not needed though this is easily 100% reproducable.