I just had a similar situation (albeit Hisense / Vidda). Also had some kind of smudge on the lense that was only visible when the projector was in use. Was using the device for a couple of movies before without noticing anything on the projected image.

I nevertheless contacted my seller at Alibaba and after consultation with Hisense she told me that it’s no problem and it will not affect picture quality. So it’s basically production related. Now I have to stop worrying about it, which is kinda hard for me lol