Lightroom Photos is just a timeline-based view of all the photos in all Lightroom Collections segmented by year and that is why the synced images are added by to "Lightroom Photos" view. To stop this from happening, you should add images to a specific collections simply tap on the three dots next to that collection and select "Add Photos".And if you want to auto add images from camera roll to that specified collection select "Enable Auto Add".
Refer Get started with Lightroom for mobile app on Android for more details.
I am not sure whether that really helps. I always start creaing a new collection in the lightroom desktop app. Then I assign pictures to that collection. After that I start synchronizing that collection by clicking on the respective icon next to the collection in the desktop application. What then happens is: All photos are visible under the collections name on my phone and in addition they appear in the "Lightroom Photos" section on my phone. Even after I stop synchronizing the collection via my desktop application all pictures still appear in the "Lightroom Photos" section. My phone is NOT set to "Enable Auto Add" and I do not have a problem with adding pictures that are on my phone to one of my collections.
Lightroom mobile is just a view and should not be confused with Collections.
Try adding an image on your Desktop in a collection that is also synced with mobile.
You should find the image in the collection on your mobile once synced,and it will also show up in the Lr photos view.