OK so you have the associated dimension but the angle of the dimension line doesn't change when you move your slab.
You can realign the dimension but you need a line to do this.
So, draw a line between the two slab nodes that you associated to originally (where you have drawn in the red dimension in your image).
Now click on the dimension to bring up the pet palette.
Choose the 'Align Dimension Line' option (top middle option) and then click on the line.
You may need to adjust the dimension position and witness line length as well.
Now you can delete the line.
It would be nice if you could realign to a guide line but that doesn't seem to work.
Or even better if it automatically realigned which is what you are wanting.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10