iCloud photo library and WhatsApp - "This photo has not yet been downloaded to your iPhone"

I recently activated iCloud photo library and turned on the "optimise storage" option. All photos were uploaded successfully, but occasionally when I have tried to send a photo or video in WhatsApp (usually a slightly older one), I get the message "This photo/video has not yet been downloaded to your iPhone." Is this expected behaviour? I was under the impression that optimised (lower res?) versions of my photos/videos would have been downloaded to my iPhone, but it seems like some things have not been downloaded at all, despite the fact that they were on my phone in the first place. Is there some way to keep some photos/videos downloaded? I tried editing one video to make my phone download it but, after a while, I get the message again that the video has not been downloaded.

Can anyone offer some guidance here?

Thanks