Thanks for the advice! I run macOs Catalina 10.15.7. I also have an iPhone 11 Pro with OS 15.6.1. It looks like all photos are remaining in previous months and following months. Not one single photo exist in July for example, or in any of the months between the dates mentioned. Not when looking in Photos on my mac, not on my iPhone and not on iCloud logging in from a browser.
I appreciate your suggestions and considering them I conclude following:
1) I have used 26 out of 200 GB, so I have 176 GB available. I have not received any warnings.
2) No family or friend have access to my iCloud. I guess I would get a warning if someone managed to login from another device?
3) All photos missing were taken with my iPhone, same as the remaining photos in the previous and following months.
4) iCloud photos was enabled both before this happened and after. I'm missing photos taken with my iPhone.
5) I have not changed the Photos library, ever. I have no external drives connected.
6) In photos on my mac I tried both to go to the year and month and I have also tried with click on "All photos" where they all look to be in date order. Going to the actual period there is not a single photo between these two dates. Same if I login to iCloud photos by a web browser. The photos before and after exist but all in the same period are gone. No hidden photos and not in last deleted.
Not sure I dare to say it but maybe iCloud is not perfect, maybe there is a defect in the system? I would love someone to prove me wrong.