Detected during testing #1503.
Also reproducible @ 9f4afd6 develop
Tested with iPhone 8 plus, iOS 13.1

Reality:

• Add media & text block, tap placeholder and choose Choose from device
• Select a video
• Start upload but before it finishes close the post by selecting Update Post
• Re-open the post before upload finishes
• Wait until upload finishes
• Video is shown as blank
• Switch to html editor
• An image url is seen instead of the video url, and that image url is the cover url of the video
• Lastly, when you open the WPMedia library you can see that cover urls are uploaded there unintentionally:

Expected:

• Add media & text block, tap placeholder and choose Choose from device
• Select a video
• Start upload but before it finishes close the post by selecting Update Post
• Re-open the post before upload finishes
• Wait until upload finishes
• Verify that video is shown properly
• Switch to html editor
• Verify that video url should be seen
• Lastly, verify that cover picture of video is not uploaded to WPMedia library