As reported in the Moodle.org forums in English and Spanish, since the 3.11+ upgrade of 11 June, students see the link "Add an activity or resource" and teachers see it without the editing being turned on. When they click it they see an empy activity chooser, but they should not be seeing anything at all.

Also, note that the "Add an activity or resource" links are shown no matter edition is turned on or off. Only should be available for "teachers" and on edition mode.

Edited, there is a proposed patch @ moodle.org's forums: https://moodle.org/mod/forum/discuss.php?d=423427#p1705918

Note: This issue only affects 311 as master has already moved on from this, but the patch should be applied to both to allow contributed course formats to function correctly.