Android phone automatically deletes photos?
Photos could be removed by a cloud sync/back up application. 

One example is Google Photos. You may have chosen to remove photos which have already been backed up. You may not see them in the Gallery, but they will appear in Google Photos thumbnail.

Photos are removed from your device by tapping menu -> Free up space. 





If you did, you can still see your photos, including the ones removed, in the Google Photos app and http://photos.google.com.