tab/spaces setting "mixed" only generates spaces.

I'm using Qt-Creator 3.1 branch of today and use fakevim as an editor component.
I created a new codestyle for my company and choose to use the "mixed" tabs setting for whitespace.

I notice that if I create a new line, or update any line using the tab button my editor shows tabs during navigation. Until the moment I press save, then the leading whitespace is replaced with spaces.

In the coding style edit dialog I use "tab policy: mixed". Both tabsize and indent size set to 4.
I tried all the options for continuation lines, they didn't change the outcome.

Create a new empty line inside a C++ method (in a .cpp file), type 2 tabs and then some code.
Press save.

I noticed that the whitespace I typed using tab is now replaced with spaces.

I expected the "mixed" to use tabs for indent.

ps. the preview text-area in the config dialog doesn't change its content when switching to mixed, seems like a bug too.