This has been present since X1 but I only reported it recently. If you want to reproduce the behavior, follow these steps:
 
1. Load the Normal project template.
2. Hit Shift+B to hide the Bus pane.
3. Adjust the height of the MultiDock so that you can see only one track (Track #1) in the Track View, like this: http://i.imgur.com/MN3eV7z.png
4. Hit Shift+D twice (the MultiDock is maximized and then restored, as it's supposed to).
5. Hit D twice (the MultiDock is minimized and then restored, as it's supposed to).
6. Hit D once, followed by Shift+D twice.

[Expected Results]
The MultiDock height is restored like the screenshot in Step 3.

[Actual Results]
The MultiDock height is initialized.
 
Reported on Dec 6, 2013
Confirmed and submitted to development on Dec 13, 2013

Can reproduce. Thanks.

According to Jlien X not fixed in X3E.

I first noticed this several days ago and.........YES! This is definitely fixed in SONAR 2015!
Never expected this minor but annoying issue to be fixed, to be honest. Thank you, Bakers! 

Interesting, that wasn't even mentioned in Noel's new blog post about bug fixes so I'm glad you did.

Developers have a nasty habit of fixing things without telling anyone :)