We just started experiencing a similar issue yesterday (7/12/2019). After much head scratching, we found that removing the generic usb memory stick we had in the back of our Roku TV, which is used for recording/playback for the on-air antenna tv channels, cleared up the problem, the channels then displayed properly. It had been working fine for the past 8 months or so, and suddenly yesterday afternoon after turning on the TV, noticed the antenna tv channels showed a blank screen, (although the channel info at the bottom of the screen still displayed, just no picture).
We suspect it was/is in fact a software update issued with Roku, since the memory stick is fine, it can still actually record and playback the live on-air channels for display, so we know its receiving a signal, but cannot in fact display the on-air channels live. Once the memory stick was removed the on-air channels displayed fine. It appears that for some reason, the latest Roku software is preventing displaying of the live on-air channels.