The union of parametric features tool in connect currently leaves a lot to be desired. If two fully constrained parametric solids are merged any constraints that existed between the solids are gone. Unless they were 2D some 2D constraints persist (which is also confusing). Then you are left with the option to use 2D constraints only to re-constrain the two solids that were already constrained. To make things more confusing any "extrusions" that had variables applied get the variables dropped after the union retaining the last known value.
As a result of this odd behavior I don't even try to use the union tool if I can avoid it with parametric solids and apply imprints to solids and then use the modify feature tool to extrude the imprint.