Roy I tested this a bit. The value I send in the X-Bunq-Client-Request-Id header is always exactly the value I get back also in the X-Bunq-Client-Request-Id header. There is another header X-Bunq-Client-Response-Id that has a value like in your post, but these values aren't mixed up for me. Not sure how you're parsing the response headers, but if you're getting something in the X-Bunq-Client-Request-Id header that you didn't send, that could be a problem on your side.
As for your main request here, yes it doesn't seem to be a problem to re-use the same exactly same request ID header. So I guess nothing changed since the other thread that you linked at the end. I would agree with the sentiment that this is a bug, because the documentation suggests that this shouldn't be possible.