a starbase can also seem to dissapear (DL) because of a unit selection querk.
if you try to build a starbase using a constructor that you moved from a stack of constructors, it can appear to build a starbase but then no starbase appears! What has actually happened is that the constructor you were moving around, did not build the starbase, a different constructor back in the stack built it where the stack is!!
You have to be vary careful when selecting a constructor from a stack, you have to deselect any duplicate selections, and just to be safe, click several times on the one you really want to build the starbase.