It’s because of how Aximmetry bypasses the TAA for the Billboard. It can’t do that while supporting TAA, from what I understand. 

Hello, 

The billboard is not affected by the depth of field effect due to limitations in the Unreal Engine. Translucent objects like the billboard are rendered after the DOF blur is applied to the scene, so these objects are not blurred. There is an option to render translucent objects before the DOF is applied by disabling "Render after DOF". This blurs the billboard, however, it leads to severe rendering artifacts (especially at the edges) because the depth information written by the billboard is not accurate enough for a good quality DOF calculation. For this reason, applying DOF to the billboard is not supported.

Best regards,