Issue description
When trying to add another text line below the dimension value with the dimension text settings, it seems that the "Anchor" field is grayed out and un-editable.
Solution
This is a limitation. It works fine with the standard linear dimensions, but not with the elevation dimension or the level dimension
In the Dimension Text Settings, go to the Text Style tab and uncheck the Always Readable then click OK.
Open the Dimension Text Settings again and you can now edit the Anchor option.