Sorting & grouping defect

I implemented outlook-like grouping by time. Groups: Today, Yesterday, ...
When grouped by time column and the column is visible in grouping area as well as in columns header - sorting inside groups is broken.

Sort direction is synchronized between group box column and header line column. So when i click on column in group box area, the sorting arrow changes in header area for that column. But rows inside groups are not sorted as they should - by time column. But when I click on time column in header area - it does what it's supposed to do, sorting mode is not changed, but rows become sorted.