Navigating a slightly awkward fandom etiquette situation that I'd be interested in other folks' input on

I was a part of a really neat little gift exchange where my own prompt ended up having to be filled by a pinch-hitter, which may be one reason for the mismatch. The TL;DR is I had requested a genfic (unsurprisingly to you all, I am sure) and the gift I ended up getting was... not, and felt a bit like it veered from the prompt to additionally focus on a character I don't care as much for.

(Ultimately I am not horribly put-out because this was for a small fandom and there were a number of other works people did for the exchange that I got excited about, and also the vibes of the whole event have been lovely and kind which is why I don't want to kill that!)

My dilemma is I feel like I still ought to leave a comment on the fic -- it doesn't seem right not to on an exchange gift, especially when someone stepped up to write something last minute and I think just genuinely didn't understand my preferences. However I am still feeling a tiny bit of aromantic weariness about the situation, and feel a bit uncomfortable with that. My usual "If you can't say something nice, don't say anything at all" fic policy is failing me here, and there are at least a few bits of it I still liked and could bring up in a comment. But it all feels a little disingenuous.

Just contemplating the best sort of comment to leave that won't hurt the writer's feelings and even expresses some gratitude for stepping up at the last minute (since I'm sure there are other folks getting excited about the story, and I like that thought) but also doesn't totally misrepresent what I am About in fandom, you know?