It's difficult to work out why # shows in PrimeNumber7's profile. Add or remove DefaultTrust to trust settings doesn't affect it because # stays. It doesn't affect me when I'm online. Some users see it's there some don't so if theymos posts what's behind it I'd like to know.
Your question has already been answered. The "#" shows by default because of the the flag that QuickNumber has. You can verify that by adding ";dt" to the URL, e.g.:
https://bitcointalksearch.org/user/primenumber7-2561166
Users that don't see it most likely have custom trust lists that include more of the users that oppose the flag, or exclude more of the users that support it, compared to default trust. They can also verify this by adding ";dt" to the URL, liked decodx did.
There is no bug or anything else for theymos to address.