I can attest to the PITA that unfolds when you cancel a return, particularly international.

Summer 2022 I cancelled the return on a US-AMS trip, and ended up returning to the US on a different ticket. The credit from the cancelled ticket was trapped, and only usable on a trip from Europe to the US. I had a trip in spring 2023 to Germany, and ended up booking a mileage ticket on AA for the US-FRA. I was able to use the earlier credit for BER-US. That return trip did not actually use all of the credit, but by flying that BER-US flight, it released the residual into a new credit that didn't have the original restrictions (and which I was then able to apply to a purely domestic US trip later in the year).

This all worked out, but was tricky to figure out.