BitBucket Site does not remember last used account correctly

I have two accounts, 'st3fan' and 'arentz'. The latter is a paid account. When I log in to the arentz account, close my browser and come back after a little while, I am greeted with 'Welcome back st3fan'.

My browser is Safari. I've tried to delete all cookies from bitbucket.org but it does not help. Does Bitbucket use some other mechanism to identify the last used account? If so, it does not seem to work correctly with multiple accounts.