Hi - I’ve seen this asked a few times when searching and usually a Shopify Guru asks to reach out direct.
I have a few products that just won’t stick in any collections. I add them from the Collections Page and I also try adding from the Product Page. It looks like it works but after saving and visiting the page, it mysteriously disappears from the collection. Only happening with a few new products from what I can see.
Just after saving:
After Refreshing:

