Photos are imported but I can't see them

I have 2 iPhones and a lot of photos on them. Few months ago I experienced the problem described in topic Photos can't see all images on my iPhone for importing. I found that naming convention for photos (IMG_XXXX.jpg) allows only 10000 different photo names. Photos app marks photos with the same name as "already imported" in spite of different date / place and of course contents.

Now I importing photos to my library this way:

1. I download all photos from both of my iPhones using Image Capture app to some local directory of my MacBook Pro

2. Rename all photos with some naming pattern: YYYY-MM_XXXXXXX.jpg to be sure that I will not have name collision

3. Import renamed photos to Photos.app using File > Import... dialog

Last time I imported about 4700+ photos from my last trip. All of them was imported successfully, but I can see in my photos / moments list only a part of them. If I select all photos that I can see and do File > Export > Export unmodified original... to some folder I will get about 2500+ photos instead of imported 4700+.

Today I opened Photos.app again and during startup loading I was able to see some all photos from my last import in "Moments", but after few minutes when Photos.app has been completely loaded these photos have disappeared again!

If I try to make File > Import... again, Photos.app will show me that ALL of the photos are already imported. And I know that all these photos is in my library, but I don't know how to get access to them!

I have no hidden photos / or photo albums.

How can I reach ALL of my photos and why Photos.app hide my photos from me?