When I get accepted from 2 different subtasks from different submissions (in same problem), my score should be calculated as the sum of these subtasks.
But, the site calculates my score as the maximum of those submissions.
Not a bug but a feature, sorry.. Note that this website was implemented before IOI 2017 where that scoring was introduced, and it is hard to reimplement all those stuff
When I get accepted from 2 different subtasks from different submissions (in same problem), my score should be calculated as the sum of these subtasks. But, the site calculates my score as the maximum of those submissions.