Kontact seems to do some magic regarding his initial window placement. Typically KWin remembers the last size and position of a window and restores it, when it is opened a second time. This is not the case for kontact. Furthermore, windows rules that are applied initially for the size or position of Kontact do not have any effect (using rule mode force does have).

Thus, I guess Kontact does some special handling about the size and position of the window and does some magic about its placement on its own (and bypass KWin rules by doing so). 

This is also problematic if one is working with dual monitor setup as Kontact is always started on the left screen ---regardless of the primary screen setting or the active screen---. If the left screen then has some horizontal offset, the position of Kontact may be even outside of the visible area!
Example: I have to monitors and can only move Kontact to the visible area by using a shortcut. It is always started on screen HDMI1.  As you can see from the screen configuration below HDMI1 has an offset of 416 pixels, which is not respected by the Kontact window placement.
Screens: 
- HDMI1 connected 1280x1024+0+416 (normal left inverted right x axis y axis) 380mm x 300mm 
- DP2 connected primary 2560x1440+1280+0 (normal left inverted right x axis y axis) 600mm x 340mm


Proposal: Do not set the size and position of Kontact after initialization. This breaks the placement rules of KWin.