Did you do all the Settings in unreal correctly? We had the Same bug, when we swaped the Virtual Camera with a Tracked one 

Hi Rho, 

We've discussed this in email, but I wrote it here down as well for common benefit.

You are probably in Free camera mode. In this case you won't see acceptable picture on the billboard, because it is only projected correctly when you look it from the actual tracked camera position. Using free mode is only necessary if you want to fine adjust the position of the billboard, e.g. when it is placed right behind a table.
Normally you simply have to use the Lock To Camera feature of the billboard while you positioning with the SCENE / Base Cam Transf. Please follow this description.
https://aximmetry.com/wiki/index.php?title=Setting_Up_Virtual_Sets_with_Tracked_Cameras#Talent_coverage:_using_the_billboards