Custom Fields Don't Show in Setup for Advanced Search

Hi – I have a weired problem:

I created 3 custom fields “field-of-interest”, “activity-month”, and “activity-year”.
I first set them up in Listing-Setup.
I set them up in General Setup -> Registration.
I then set them up under General Setup -> Advanced Search, and when I opened the tab for “Field of Interest” field-of-interest in the dropdown menu of “Search Field”

So far everything went fine!

Ok, then I did the same with one more custom field:
I set them up in the Listing-Setup.
I set them up in General Setup -> Registration.
But now it doesn’t show in the dropdown menu of the Search Field of General-Setup -> Advanced Search.

What did I do wrong? What did I forget?

Best
Uwe

(I have to say this was my second attempt. I did it before, it didn’t work, then I checked the forum and found out that changing custom fields is not recommended – which I had done – so I started anew with a new WordPress installation after emptying the database … so my problem here occurs with this new installation)