Exported Video Won't Play Back on iPhone/iPad

Hello! I've loved using DaVinci Resolve & currently use Resolve 16.2.8.005. However, this has been a problem for a few months. I can record my videos on my Canon EOS R, my old iPhone 7 or my current iPhone 12 Pro. Resolve edits them just fine, and I regularly upload to Twitter, YouTube, Vimeo, etc. But as soon as I transfer it to my iPhone 12 or iPad MUUJ2LL/A, the files don't play.

I'm used to using the standard output for YouTube 4K or 1080p, or Vimeo 4K or 1080p. All these used to work, but no more. All these setting are H.264, so I'm not even using anything like "advanced" technology that might not be compatible. I'm using a 15" MacBook Pro from 2018. The files all play back fine on this laptop, just not on either iOS device. It was working on the iPhone 7.

The video is black screen with audio, but with a still image from the video now and then.

I thought I'd find the answer for sure if I searched the web, and searched this forum. Felt certain others must be having the same issue, but not finding it. Thank you, especially if the answer is something obvious to you.

There must be a bug, but the workaround seems to be switching off the hardware acceleration while exporting.
Annoying nevertheless.

Thank you so much.