Issue Description
After modifying the Partial Structural Display to show only the "Core" so the dimension could be added. When switching it back to "Entire Model" the dimensions disappeared.
Solution
Dimensions are associated with elements. This is why they disappear if you change the partial structure display.
Dimensions in Partial Structure Display
Each Partial Display mode can have its own set of dimensions: you can have a set of dimensions in your Core Only view, and another set of dimensions in your Entire Model and Without Finishes views. If you switch between Partial Structure Display settings, the dimensions associated with the earlier Display setting will not be visible. To see the Dimensions again, switch back to the Partial Structure Display setting in which the Dimensions were placed.
This wall is displayed with the Core Only display setting:
If you now switch the display setting to Entire Model, the dimension shown in Core Only disappears, and the other dimension - the one previously created with the Entire Model display setting - appears.
You can copy-paste dimensioned elements between two views whose Partial Structure Display settings are dissimilar. However, if you try to paste dimensioned elements that do NOT appear in the given Partial Structure display, those dimensions will be lost after you paste.