iOS - PhotosKit - Troubles identifying modified assets

Question

I'm working with the Photos framework, specifically I'd like to keep track of the current camera roll status, thus updating it every time assets are added, deleted or modified (mainly when a picture is edited by the user - e.g a filter is added, image is cropped).

My first implementation would look something like the following:

private var lastAssetFetchResult : PHFetchResult<PHAsset>?

func photoLibraryDidChange(_ changeInstance: PHChange) {
    guard let fetchResult = lastAssetFetchResult,
    let details = changeInstance.changeDetails(for: fetchResult) else {return}

    let modified  = details.changedObjects
    let removed   = details.removedObjects
    let added     = details.insertedObjects

    // update fetch result
    lastAssetFetchResult = details.fetchResultAfterChanges

    // do stuff with modified, removed, added
}

However, I soon found out that details.changedObjects would not contain only the assets that have been modified by the user, so I moved to the following implementation:

let modified  = modifiedAssets(changeInstance: changeInstance)

with:

func modifiedAssets(changeInstance: PHChange) -> [PHAsset] {
    var modified    : [PHAsset] = []

    lastAssetFetchResult?.enumerateObjects({ (obj, _, _) in
        if let detail = changeInstance.changeDetails(for: obj) {
            if detail.assetContentChanged {
                if let updatedObj = detail.objectAfterChanges {
                    modified.append(updatedObj)
                }
            }
        }
    })

    return modified
}

So, relying on the PHObjectChangeDetails.assetContentChanged property, which, as documentation states indicates whether the asset’s photo or video content has changed.

This brought the results closer to the ones I was expecting, but still, I'm not entirely understanding its behavior.

On some devices (e.g. iPad Mini 3) I get the expected result (assetContentChanged = true) in all the cases that I tested, whereas on others (e.g. iPhone 6s Plus, iPhone 7) it's hardly ever matching my expectation (assetContentChanged is false even for assets that I cropped or added filters to).

All the devices share the latest iOS 11.2 version.

Am I getting anything wrong? Do you think I could achieve my goal some other way?

Thank you in advance.