Sounds like an unorthodox order entity. Usually, the order’s items are stored in a separate entity with a 1-* association for order-orderitem. Your version has a self-reference. Can you share a screenshot of your domain model?
The reference selectors on your page probably refer to the same association. Which causes them all to change, when you change one.
As Tim mentions you need a orderline entity
Instead of a single reference selector, you need reference to multiple products. But in a reference you cannot store a additional information, like the quantity etc. So you need a entity in between, in this case orderline.
How to create the right pages and references, please take a look at this learning path: https://academy.mendix.com/link/module/93/lecture/2799/6.7-Information-Entity
Maybe starting at the very beginning of this learning path will be even better to understand the Mendix basics.