Update. I filed this as WebKit bug 323550 and Apple triage has marked it REGRESSION(? - iOS 26.6) and added it to Radar. For anyone tracking the version: WebKit's MP4 recorder has written a rotation transform into the file since a 2020 fix (bug 198912), and Apple's own commit for the WebM rotation fix in April 2025 still describes mp4 as carrying that metadata, so iOS 18.4 is the last version where it is documented as present. Reports of it missing on the front camera start in June 2025 on thread 786803. I reproduce it on 26.6. The sourced timeline is on the bug.

Same family of bug, seen from the recording side, on iOS 26.6. The front camera track reports the sensor buffer (1920x1080) whatever way the phone is held, and neither getSettings() nor screen.orientation will tell you how the frame is actually drawn, which matches what you are seeing with screen.orientation stuck on portrait-primary in home screen apps. What worked for me was to stop asking the APIs and measure the picture: draw one frame of the video element onto a 16 by 16 canvas scaled from the larger reported dimension, and check which corner has paint. Bottom left painted and top right not means the frame is tall. From that you can rotate the preview yourself, or in my case decide whether to record the raw track or a canvas of the drawn frame. Write-up with the dead ends: https://dev.to/lagudafuad/why-your-web-teleprompter-records-sideways-on-iphone-and-the-fix-549m and the code, MIT: https://github.com/lagudafuadtosin/web-teleprompter (src/lib/camera.ts). The recorded-file half of this is WebKit bug 323550, which Apple has triaged as a regression, and my own thread here is 844826.

Still happening on iOS 26.6, and Android Chrome does the same: the track reports 1920x1080, the drawn frame is portrait, and the file has no displaymatrix. Two things worked for me. Request the camera in landscape numbers (width 1920, height 1080), which makes Safari rotate the picture itself. Then probe the drawn frame by drawing one frame of the video element onto a small canvas and checking which way it extends, and if the track and the picture disagree, record a canvas stream of the picture rather than the raw track. Working code, MIT: https://github.com/lagudafuadtosin/web-teleprompter (src/lib/camera.ts).