Same thing happened to two of my contacts on November 9th. I used to be able to see their last seen up until around 3 pm that day before both their last seen disappeared. Their last seen was removed both at the same time. I’m positive that they didn’t block me or delete me as a contact or change their settings both at the same time. That would be extremely rare. I’m sure it has to do with the new privacy settings put in place by WhatsApp. The common factor between the two contacts is that I had sent them both a message around 1-2 months ago but neither had responded to me. I think WhatsApp is removing the ability to see last seen for contacts who have their last seen visible to “only contacts” and who you have never had 2 way communication with in the past. With that said, can you let me know if you ever contacted your contact in the past, and if so, did they ever reply?
It is a disaster with their security and unreliable, I think you have to id manually to see if you really got blocked, for instance:
you cannot see their last seen or chat window, I also think that their profile picture is not updated. Oh and I think one factor that will confirm everything if your voice calls don't go through. It took me a while though but yeah check those factors.
If you've been blocked by someone on WhatsApp, you'll no longer be able to see their last seen or online status. Calls and messages may not work also.
What @evelinchamplin tried to explain is that you'll continue to see their profile picture, but if they decide to change it, you'll not see the new one.