iCloud Photo Library

Hi,

I have around 8000 photos in my iCloud photo with the majority stored in the cloud and ‘optimised for storage’. However, occasionally when I look at my library...the total number has decreased to 4000.

I previously resolved this by restoring my iPhone from a previous back up but I don’t really want to keep doing this as it means all of my apps etc. need to be redownloaded which is time consuming.

Does anybody know why this happens and am I at risk of losing any of my old photos from the iCloud Photo Library e.g. if my photo count drops from 8000 to 4000 and then my phone backs up to the cloud overnight...will 4000 photos be erased from the cloud?

Many thanks for your help!