[O] Can't accomplish agenda sorting '(priority-down category-keep)

[yurkobb]

Dear org-mode community,

I have a problem sorting my agenda in the following way: group by
priority descending, and withing each priority (A, B or C), sort by
category as configured in org-agenda-files.

My org-agenda-sorting-strategy is set to '(priority-down category-keep).

What I see in the agenda instead is that tasks within a single priority
level are sorted not by category, but by some other criterion (looks
like the time left to their deadline). In fact, tasks from the one
category appear interleaved with other categories' tasks, within a
single priority. This is all within the tasks that don't have a priority
set explicitly, i.e. should be the default B priority.

Interestingly, the order doesn't change at all if I swap the
"category-keep" criterion with "category-up" or "category-down".

Any hints on what may be wrong?

Thanks,
Yury