So, I initially modified this work: https://musicbrainz.org/work/91330cf4-fc1d-4a6b-807b-667f6b4c9b40 to add the "Is Based On" relationship for three other works... And then later, I decided that the ordered "Parts" relationship would actually be better to use... So I edited the https://musicbrainz.org/work/91330cf4-fc1d-4a6b-807b-667f6b4c9b40 work... clicking on the little pencil icon for the first "Is Based On" relationship, modified it's type and direction to be "Part", and clicked "Done".
The pop-over editing form/window did not close, but in the background, you can see the work moved into a new "Parts" grouping. Clicking "Done" a second time closes the form/window.
I'm not sure if this is a second unrelated bug or not... but I did the above steps three times, for each of the three works. After the third, there was only one Work now displayed; I clicked "These relationships have a specific ordering"
I tried adding one of the invisible works to this new relation, and it appeared as item number "4.", so I deleted that... and then hit save to see if this would correct itself...
Oops, something went wrong!
Time: 2019-05-03 20:21:59 UTC
Interface language: en
We're terribly sorry for this problem. Please wait a few minutes and repeat your request — the problem may go away.
If the problem persists, please report a bug and include any error message that is shown above.
Please be sure to include the stack trace and request data shown above!