Steps to replicate the issue (include links if applicable):
What happens?:
In the section "Test Results", we can see the implementations heading the columns. There are two columns for the two implementations, one saying "successor in JavaScript" and the other "Current implementation". It seems that is the case when the implementation has no label.
What should have happened instead?:
It should show the ZID, i.e. Z10231, instead.
Software version (skip for WMF-hosted wikis like Wikipedia):
Other information (browser name/version, screenshots, etc.):
Adding / removing the label indeed exhibits that behaviour.