I would be a little careful with United and their Economy Fully Refundable tickets. I booked the same recently and had a difficult experience with the refund process. It worked out in the end, but not without a dispute placed with Amex against United.

• Booked a return ticket from LHR on KKFFDXU5 in June 2022. Before I booked, I noticed the same thing you did ("Fully Refundable" on one page, but "Non-Refundable" on another page). I called United and the rep took some time to ask around and confirmed what I was about to book was truly a fully refundable ticket.
• Within a couple hours after booking, made a date change on the return leg, keeping it under Economy Fully Refundable, but with new code QKFFDXJ5.
• A month later, tried to cancel online, but the website only offered a full refund as future flight credit. So instead, I called United to cancel and refund to my credit card, which they obliged. (FYI I've since rebooked a new and similar itinerary on the same original fare code, and the website again lists my flight cancellation option only for future flight credit.)
• A week later, I receive an email confirmation from United stating they had refunded me minus a £190 cancellation fee.
• Spent hours upon hours arguing with countless United reps on the phone. Turns out, when I made the change to my return leg, United inexplicably changed the fare code of my outbound leg to the non-refundable regular Economy KKRCDXU5!
• Some of the reps acknowledged my outbound fare code shouldn't have changed, and that I should have been clearly informed if it did. But despite that, none of them were able nor willing to refund me. Per United, my only course of action was to submit a complaint on their general feedback page.
• Finally submitted a complaint to Amex, attaching a PDF of the fare rules of the original Economy Fully Refundable flight I booked, with largely the above details. Amex closed my case and credited me the £190 within a day or two, with United refunding the rest.

This could be specific to the fact that I made that flight change. But still, tread carefully!

One thing worth noting is that it seems these Economy Fully Refundable flights are only offered when their website is set to certain countries. When you toggle the website to the US version, you don't get the Fully Refundable fare class. Not sure why they offer these tickets only to certain markets. But seems to me their systems and processes are not setup to properly deliver what they're selling.