Here's what support said. Basically what you said.
I have checked the reported behavior and noted that this is a standard behavior. When you unbuild the related Assembly Build of a Work Order, the status of the Work Order does not go back to Pending Built. You may notice that the Unbuild transaction is not linked to the Work Order and the Assembly Build, only the Assembly Built is connected to the Work Order, this is the reason why the Work Order still shows as Built.
Since the unbuild transaction is not linked to the Assembly Build, there is no option to rebuild. The work around you may need to do is to create a new Work Order for the Assembly Item and then create the Build.
An enhancement request is already filed regarding the linkage of the Assembly Build and Assembly Unbuild. I will attach the case to the enhancement for this to count as a vote and I will also send you a separate email containing the details.