Eggs collapse on the bottoms only?

One of my clutches looks plump on day 50, only the bottoms of the eggs are collapsing up, into the egg, in a concave shape. Has this happened to anyone before?

All my clutches this year are in the same incubator as this clutch, except, the other clutches had the typical collapse from the top down, not the bottom up, haha.

Thanks!

It could be a humidity issue. I know of one example that happened much earlier on where they dimpled on the bottom early on because the new substrate being used wasn't holding moisture. Those eggs didn't survive, but at day 50 you're already expecting dimpling so I imagine it's not going to be as detrimental.
Try adding a little more water, see if that prevents it from getting worse.