I'm rather confused by the flat-earth thing as well. Although I'm not sure its correct to attribute this to millenials, ages 18-24 are generation Z or "Zoomers" as they have been called.
Millenials are ages 25-39 or so, and I believe the vast majority of them are round-earthers.
All the same, like this flat earth event or not, questioning scientists and holding news agencies under tough scrutiny is a very good thing. Maybe there is no harm in the older generation needing to defend seemingly obvious or fundamental topics. Why not keep asking why the sky is blue or the earth is round?
Sometimes, when we are forced to defend points we've taken for granted it makes us look at a subject again and learn something new. How can this flat earth issue be resolved? I would say with a calm, polite and reasoning response.