Completion: Form locking is still not working right

This sequence demonstrates the problem:

0. Enable completion at system and course level.
1. Create a new Forum. Set it to be automatically completed when user makes 1 post/discussion.
2. Save and display. In the forum, make a discussion.
3. Edit settings again. Observe that completion settings are now locked because one user (you) has completed it.
4. Click the unlock button.
5. Observe that on the reloaded form, completion settings are now enabled and appear correctly (with the 'require 1 post/discussion' ticked).
6. Save and display.

EXPECTED: It should save the changes (clearing completion status, but basically no change).
ACTUAL: 'When you select automatic completion, you must also enable at least one requirement (below)' error.

7. Edit settings again.

EXPECTED: The settings should be as before (require 1 post/discussion). They'll be locked again.

Seen in 2.6.2 and in current master. (Might also affect 2.5, unknown.)