I visited improving.duckduckgo.com and they are linking to this article[0] that explains how this domain hosts an "anonymous tracking pixel" (that respects DDG privacy policy) to collect data on how DDG is used (feature usage, A/B testing etc)
But it does not tell how it could end up on other domains. Are you sure it is not part of the DDG result page or extension ?
I use uBlock so I do not check every request.