P: Photos disappear in my Album, but the number is there I can see the pictures in "All Photos"

I use Lightroom on Mac Pro, BigSur 11.2.3, I have all my pictures on the external hard drive and do not use the cloud.

Next to the album, I can see the number of pictures that were there, but the screen of that album says "This Album Is Empty" add photos etc.

It is the second time that it happens. I can see the pictures in the folder and in "All Photos" so this is not an access to the hard drive issue.

What is happening?