Roku Ultra not playing MP4 file from thumb drive after update

Did it play with any of the 14.x updates?  I think the first one was 14.0.4.  They changed something with the way audio is handled, so you might try changing the audio settings and if you're using any of the Volume modes, try disabling them.  You can find them under the settings you get when your press the * button while a video is playing.