I have read some cannot use MulitView (Sportsbar Mode) with U308. I have tried it a couple of times and have no trouble what so ever. I have tried it while viewing a recording on my DVR, having an OTA channel playing and changing channels and have no issues.
Do you have a 4K TV? It works for me in 1080, but not 4K. It worked fine in 4K on 307.