Hey there Nole81,
We understand you're trying to keep the Messenger app in landscape mode, but when you use the orientation lock button, it reverts to Portrait. We'd be happy to provide information about this.
Keep in mind, the orientation lock button in Control Center locks orientation to Portrait:
Rotate the screen on your iPhone or iPod touch - Apple Support
Rotate the screen on an iPhone or iPod touch with a Home button
1. Swipe up from the bottom edge 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 or iPod touch sideways.
If the screen still won't rotate, try another app, like Safari or Messages, that is known to work in landscape mode.
If you want Messenger to stay in landscape, do not enable that option.
Kind regards.