This is related to an ongoing bug, where a blade already part of a weapon reattaches to another grip/guard. The resulting sword has no value associated with the blade, whereas the remaining grip which had its blade removed retains the value of the blade. (Non-sword weapons also have this bug)
If you're clever, this bug is quite exploitable. Performing the bug again (attaching the valueless blade to a new grip) will give the blade's value to the previously connected grip.