Several users discovered and pointed this out today on the Forums at https://community.secondlife.com/forums/topic/533296-dashboard/
When a friend has your "Friend can see when you're online" permission disabled in the viewer (Contacts>Friends), this friend will show offline to you in the viewer as intended, however on the Friends panel on your Dashboard, this friend will not show at all, as if they removed you from friends.
I confirmed this with a test alt after reading the comments in this forum topic.
This is a rather big problem, since this way it can be seen clearly if a friend has hidden their online status from you, comparing your friend list on the Dashboard and in the viewer. Notwithstanding trust issues, this behavior should be consistent and a friend with hidden online status should simply show offline as it used to be.
Repro:
1: Disable "Friend can see when you're online" for your alt in the viewer.
2: Wait 1 minute.
3: Open https://secondlife.com/my with your alt.
4: You will be missing from your alt's friend list entirely on the Dashboard, while you will show offline in the viewer.