This is what selected collection for a product looks like for me - there supposed to be “in collection” text there, if I remember correctly. Minor bug, I can still add or remove the item from the collection.
I’m using iPad Pro, iPadOS 26.6.1, Safari browser.
When I first noticed this, I thought it was changed for those who have a long list of collections to scroll through. It’s easier to find which collection when scrolling fast with the solid green. I’ve found it helpful and this is only a guess on my part, why the change.