When you've made your change request to add the mintage you've deleted the ND and start (1990) and end date (1990). I saw it so I've only ticked the box with the mintage. But afterwards I apparently clicked, out of habit, the “Accept all” button in stead of the “Accept checked lines” button and so the ND (1990-1990) disappeared and was replaced by a “?”.
Not quite right. I removed the ND and both dates to write 1990 instead of ND. But the Numista website wouldn't let me do that. It's a bug. $50 coins: 1988, 1989, and 1990 are identical.