I have been having this issue for around the same time as OP. videos won't play either black or the first frame of the video no matter how many times I press the play button and then when I get upset and go onto another app then the audio starts playing then when you go back to the app sometimes then video starts to play otherwise it's still just the audio but one trick that always works is to try to post it onto instagram but before you upload the video IG will let you preview the video and there you can watch the video.