Thank you for your reply, and info.
Sorry I don't know what the cause of the problem is.
Could be due to a setting or such, a photo file-type iCloud doesn't accept, photos older than 30 days, or it may even be a software fault.
Seems Photostream uploads your last 30 days and up to 1000 photos.
Photos in My Photo Stream are saved on the iCloud server for 30 days.
After that, the photos are removed from iCloud.
Apple says ... "To save or back up these photos, you must save them from My Photo Stream to your iOS device. "
If you're missing photos or videos in the Photos app - Apple Support
Says:
If you hide photos and videos, you'll see them only in your (Photos) Hidden album, so it might seem like they're missing.
More info:
About iCloud Photos and My Photo Stream - Apple Support
If you wish: you can send feedback to Apple:
They likely will not reply, but if need be, it may help make things easier in a future update
There's a webpage for sending feedback:
Product Feedback - Apple
All the best :-)