Clearer steps to reproduce (from clean MOODLE_25_STABLE install):

0. In advanced features, turn on completion and conditional availability.
1. Create new course with default settings except turn on completion.
2. Create a new Page in the course with default settings.
3. Create a new Label with default settings except under 'Restrict access' set it to require the Page to be complete. (Leave it as 'show greyed out' when not available.)
4. Enrol a test account as student in course.
5. Log out.
6. Log in using the username/password of the test account.

EXPECTED:

The label is greyed out and there is text that says it isn't available until you complete the page.

7. Click the button next to the Page.

EXPECTED:

The label is no longer greyed out and the 'not available yet' text disappears.

BEFORE FIX:

The text disappears as expected, but the label remains greyed out.