I just set a score on this map beatmapsets/372200#osu/815219. I got the full 778 combo, but did not receive the medal, despite the map being exactly 6.00*. It was no-mod as well.
Hi! Although the map shows 6.00 stars difficulty, it doesn't mean it is exactly 6.00 stars. What I'm trying to say is that it could be that the exact star rating is 5.9999, rounded up to 6 stars - meaning it is technically a 5 star.
Try to get a full combo on 6.01 stars at least or above.
You probably missed some sliderends, to get the full combo on that map you need to have 631 combo (you can see if you have a full combo if the combo number is green on the website)