Where are you located, I suspect not the US since you mentioned shell grit. Here I'd suggest you look in a feed store like Tractor Supply for oyster shell, not sure how that works in other countries. But I'd ask where you buy feed.
Oyster shell on the side, and a good layer ration with at least 3% calcium.
That happened to me and I was slow to react as at the time I was new to keeping chickens. You want to get that sorted ASAP because if they step on those eggs and break them, they find out eggs are delicious. Then that hen shows the other hens what wonderful snacks eggs are and then your flock now has a major issue with eating their own eggs.
Ended up having to butcher the egg eaters which was sad, as I like my chickens and had no interest in raising them to eat. I blamed the food at first, which may have had something to do with it as it was a small local mill. But sometimes its a genetic abnormality that is not rare, especially for the high production hybrids.
If the addition of calcium doesn't fix the issue, you may have to try and find out which one(s) is dropping the bad eggs and remove her from the flock. If its all of them, then its likely they have a diet that's very low in calcium.