White circle during photo browsing

Hi there,

I'm using the iCloud photo library with 24.268 photos. 939 of those are marked as "Favorites". There are no videos in the library.

My MacBook Pro (512GB SSD, 250GB free) is set to "Download and Keep Originals" and holds copies of all photos in the photos library.

My iPad Air 2 (128GB, 38GB free) and my iPhone 6 (128GB, 40GB free) are both set to "Optimize iPhone/iPad Storage".

I understand "Favorite" photos are kept full size on all devices if ever possible.

All three show that the libraries are udpated "just now".

So far so good. Here's the problem:

When I want to show someone a set from my Favorites on iPhone or iPad I often find the pictures in low resolution. A white circle appears in the lower right corner upon browsing. When on Wifi or LTE/3G the circle completes and seems to download the full picture after a while. Without network it stops at about a quarter.

When I let the cirlcle complete, the pcitures show up good, but may appear in low resolution again on the next day.

None of the pictures had the cloud icon with the down arror displayed before.

This is a rather embarrasing in presentation situations and about to end my love affair with Photos on iCloud for good.

Having only <1000 favorites and 40GB (one third) available memory on the mobile devices I would expect my favorites to be available at anytime. We're speaking about 4 - 5 GB which would bee needed for the favorites.

Am I missing something?

Is this a bug? Is "Optimizing" algorithm overly concerned about my free memory?

Is there a way to prevent this from happening, besides setting "Download and Keep Originals" on the mobile devices, which would basically force me to use 80GB of memory permanently for the full library ?

Is there any other way I can mark a photo to be available full-size on the mobile devices, if not using "Favorite"?

Cheers,

Robin