I use defined patterns often to create full sheets of a pattern. I use Define Pattern and have lots saved to use. I opened a couple of new ones today that I wanted to use, selected the image Edit>Define Pattern which worked, dragged it into my new workspace and NOTHING. No repeating pattern. I tried other patterns that I've previously used. Nothing. I tried white background. Nothing. Transparent background. Nothing. It won't even let me open it in the layers box--where it shows the pattern as applied--to see if it's too small (it's not, it's the same size file/sheet as all the other ones I use-12"x12" high res). These are the same steps I've been using for months and now I get nothing. Kind of frustrated because I'm behind and need to get these sent to the printer.
I am able to add a defined pattern using Photoshop 23.3.0 on Windows 10.
I'm wondering if there's either something about the document or the process where you're trying to add the pattern (maybe it's using indexed color or you have a tiny selection) or if you have an issue with Photoshop.
It might help if you could share a screenshot or screen capture a video to help show what's happening.