In LR Classic CC you sync collections not the actual image. A collection is just a List of images. When deleting them from your phone they may be deleted from LR Web and from the Collection in LR C CC but should still be in the catalog.
What you need to do is connect your phone to your computer and Copy or Move them from the phone to a folder on your computer hard drive then use LR to import them into the LR catalog using the "Add" option at the top of the LR import dialog.
I'm using a Windows PC and a LG phone and the LR import dialog doesn't see the phone Micro SD card as a drive. So I do as I stated above.
I had no idea that "All Synced Photographs" is a collection, it's listed under Catalog and that's where I was seeing things disappear. Now that I've looked at "All Photographs" under Catalog and reordered it by date, I can see the ones that were deleted are there.
I find it a little strange that when I reveal one of the mobile images in finder it shows that my mobile images are stored in a .lrdata file, where as images from a camera are stored in a folder by date. Will all future mobile images be put into there when I sync?
I don't think lightroom is capable of searching through a phones folder structure to find the images so the longer process seems to be the only way.
All synced photographs is not a collection. It is like the entry that says All Photographs. it is just a list of photos that are synced to LR Mobile (Now called LR CC) no matter what collection they are in. Just like All Photographs is a list of all the images that have been imported into LR no matter what folder they may be in or what drive that folder is store in on the hard drives connected to your computer.
No images are ever really IN LR. LR is a Database program that references your images into the LR Catalog file, a Database file, from where they are store on your hard drive.
In that same panel near the bottom is a section called Collections. Open that and you will see a set of collections. Ones that have kind of a double arrow next to them are the ones that are synced with LR Mobile. Again these are just Lists of images grouped together.
They are not folders or the actual images. Deleting, removing, images from any collection, synced or not, does not remove that image from the LR catalog or from your hard drive.