I think DDG started doing it within last 6 months. When I just switched to it, this was not the problem. Now I am almost back to using Google, because when Google omits terms, results are a bit more on the topic (when forced not to, DDG and G work similarly).

DDG always omitted terms as far as I can remember (definitely 1+y).

I can't say for sure if this got more frequent in recent times or not.