Got an update on my carrier locked ATT Note 9 about 7:00 PM EDT Friday (4/19)... Didn't see the size but only went from CSC1 to CSD1 and it shows Security Version April 1, 2019.
Yes yesterday I spontaneously got an update but it was the March 1st security update. I was mentioning it to my husband who has an S9 plus whether he got his. He couldn't remember so I was walking him through how to check. And when I went to mine it popped up with an April 1st security update when I checked for updates.. Weird as I didn't get notification it was ready and I only yesterday got notified of March 1st update.