More data on this issue (also sent to support via email; posting here for completeness’ sake, future reference, etc and so forth).
After yesterday’s test case snafu got fixed by the fabulous Chelsea and Bethany, I tried the change on a second goal. Turns out the Weekends Off feature is not the culprit (hence the change of thread title). It’s the “Commit To” rate editor. I changed my guitar goal from 5/week to 1/day. Before and after screenshots:
BEFORE
AFTER
It also derailed me, just like yesterday.
Another interesting point, related to this bug: Note how the “Final date” has had one day subtracted between the first and second screenshots.