Greetings wonderro,
To clarify, are you using the Camera app to record video or a third-party app?
Next, when you begin recording the video, is your iPhone already in landscape showing the timer at the top center and the zoom number correctly for the orientation? If not, we’d like to have you try that and see if there is any change.
Also, can you tell us if you’re watching the video back on your iPhone or via a different device?
A few basics we also want to cover and especially if this is something you’ve accomplished with the Camera app in the past.
Make sure Portrait orientation lock is not enabled in Control Center Rotate the screen on your iPhone or iPod touch
Rotate the screen on an iPhone without a Home button
1. Swipe down from the top-right corner of your screen to open Control Center.
2. Tap the Portrait Orientation Lock button to make sure that it's off.
3. Turn your iPhone sideways.
If the screen still doesn't rotate, try a different app — like Safari or Messages — which are known to work in landscape mode.
Restart your iPhone - Restart your iPhone
How to restart your iPhone X, 11, 12, 13, or later
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
Thanks.