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.