Some videos (seems random) exported from Premiere can't be Airdropped to iPhone
Digging through the thread I linked in the question (this one) finally found steps that solve it for me.
Sequence Settings
- Pixel Aspect Ratio: Square Pixels
- Fields: No Fields (Progressive Scan)
- Display Format: 29.97 fps Drop-Frame Timecode
Export Settings
- Format: H.264
- Preset: Match Source - High Bitrate
Ensuring these settings are the same in every sequence makes it so all the videos exported are able to Airdrop to and play on iOS.