I'd say this happens semi consistently to me when I get rejected. I did a tech screen (of which this is the first I've ever failed). I chose to do it in Rust as that's my main language these days. I probably should've reconsidered that decision as Rust is notoriously difficult when it comes to dealing with trees and I spend most of my time managing pointers. I described a solution to the tree problem quickly, implemented it, and then dealt with the compiler for about 15 minutes. As a result I never fully finished the task, but it was pretty darn clear I knew what I was doing.

Same thing, they came back and said no and I really have a hard time imagining why other than the interviewer not being familiar with Rust - but - hey, don't tell me it's fine to use then!

That to say, rejection feedback is almost always useless.