Environment:
Moodle - 3.9.3 (Build: 20201109)
mod_checklist - 2020012900

Replication:

1. Create a course, enrol a student and turn editing on.
2. Add a Checklist activity.
   • Checklist: My checklist activity
   • Settings / Updates by: Teacher only
   • Settings / Lock teacher marks: Yes
3. Click the "Save and return to course" button.
4. Go to the checklist activity and add three items (Edit checklist and click Add button)
   • My item1
   • My item2
   • My item3
5. Click on the Participants button in navigation block.
6. Click on a name of the student enrolled and click "Log in as" link.
7. Back in the course content page, click on the checklist activity.
8. You will see a the message:
   Note: Once you have saved these marks, you will be unable to change any 'Yes' marks

Remarks: Suggestion to change the lang string $string['lockteachermarkswarning'] or create a new lang string that will be used (to be displayed) if the user is a student.