New
#1
What worked for me.....
Dismiss Ransomware Protection under Virus & Threat Protection Settings.
HTH.
Usually when I see that in Windows Defender there is a line under indicated functions about Dismiss, frequently refers to setting up OneDrive. Since I don't use OneDrive I click Dismiss and the icon gets the green checkmark back. There's other functions that may work the same way. Just important to keep in mind that Windows Defender is more than an Anti-Virus program.
If all of the security boxes are checked and none requiring dismiss notification, run a quick scan, that should clear the yellow warning sign.