Sorry for the delay on getting back with you on this one.
I am not sure I understand what is happening for you. I tested this out on my daughter’s iPhone 8 this morning (Android user here!). I recorded a video in landscape mode and stopped the recording while keeping the landscape orientation. I then turned the phone into a portrait orientation and checked out the video. The video was still in landscape format (wide screen) and was displayed like in the example below:
What is happening for you? If you can provide a little more detail or try wording it another way, that may help. If you want to send some screenshots or a screen capture, send it over to firstname.lastname@example.org and I can get them there.