On the "new Brilliant" feed, problems sometimes have a green circle+triangle by them, and sometimes a white circle+triangle. I guess the green is meant to indicate solved problems, and hollow ones that I have not attempted.
However several that I have previously solved, and that I have previously failed, show as hollow. For example, One of them is https://brilliant.org/mathematics-problem/sequences-and-tables/?group=8rwgFNm6b7Em (I guess this may be a personalized link, but anyway).
This was a problem about 3 weeks ago. At the time I entered 2 answers which were wrong, and then the time ran out and I joined in the discussion.
I clicked on the white circle+triangle to see what would happen. It came up and showed me my two wrong answers but said I had a guess remaining. I entered the right answer (I remembered it from the discussion) and the answer was accepted. However, it then said no solutions had been posted. But there were solutions previously because I read them to find the right answer.
Also, the problem says that it is 7 months old.