I forgot about the fun of Marriott Bonvoy Gift Cards, until I needed to look up the instructions for any upcoming stay

Reporting back from past travel, Marriott Copenhagen was able to apply the gift cards in Sept 2023 - it was slightly painful process since the FD associates seem to liaise with someone in the back office to apply them, so it was a an obscured process while I was trying to leave for a flight - but they got it done

Towneplace Suites Thunder Bay in June 2023 was able to do it with some pain as well. On one stay, they accidentally overcharged the gift card so the folio was actually in a credit position of $14 (but they didn't catch it, I had to notify them). They weren't sure how to credit it back to the gift card, so they just did a cash payout. Second stay was smoother, and no issues.