I believe this is likely working as intended and there was no synchronization really, just a dangling reference to the underlying id when the literal label was removed.  It acts as a good failsafe in my opinion to allow people to avert mistakes that could cause a lot of headache to unwind.

This is different from synchronizing work item attributes to the forms that present the data, that is user controlled for the most part, requiring you to manually synch work items with new attributes added to a presentation.

In your scenario you removed a value (literal) from an attribute (enumeration), but the underlying database is still recording the previous values, it just no longer has a way to correlate that to the enumerated attribute and the work item form.  Since it is now just showing the underlying id, you could recover by editing the process xml with the old information for the value/literal xml entry (if this was a mistaken deletion) or you could use the id to build a new value/literal entry and have the work items pick it up without having to run bulk import/updates against the work items.

I may be incorrect and would love to hear what others have to say on this topic.