I have also recently ran into this issue. The reason this doesn’t work is tied to the fact that Forms are designed to capture new items/information for Monday boards. Even if you trigger an automation to send a link to someone to fill out from a current item, there currently isn’t a way to link the two back together.
I’m not familiar with how the API reads or records data for forms, so maybe there is something there?
Other than forcing you form responder(and assuming they input it correctly) to write in a specific number or text that matches your unique item, there is not native way to link form responses back to items. Plus that’s a big ask for your form responders that wouldn’t work in most use cases.
Unfortunately this topic has come up before and I think the Monday team responded with no ETA on a solution. Probably because it would require a restructure of how the Forms work, or adding a different type of form. I think a simple solution that the Monday team could implement would be to allow you to pre-populate some form fields. With that, I know you could add in a current pulse ID which would then allow you to transform the data from a response onto a specific item.
The only other way to achieve this currently would be to have the information you need sent to the pulse email for the items. Each item has specific email associated with it. You can manually retrieve said email by opening the Updates for the item and clicking the “Write Updates via email”.
If I can figure out another way to set this up I’ll let you know.
Hope this helps for now!