Photos - MyAlbum regularly changing how it sorts photos in album

I organise a large volume of photos in a number of albums, important to my job.

I have tried to repeatedly View>Sort>Newest First

Then, every now and then, it just changes this, and starts putting new photos at the bottom

(it's so weird - it maintains the previous order of 'newest first' then randomly puts the new 20 photos I have added to the bottom.)

There have been no system updates when this happens I have tried to fix and it all goes awry and I waste hours trying to get right again.

Apple is so great at so many things, the comp of choice for photographers - and yet I have so many problems with Photos. It doesn't make sense.