Behat tests fail randomly for upload/filepicker/filemanager-related feature

Feature:
In an assignment, students can upload files for assessment

Scenario:
Submit a file and update the submission with another file

Failure:
And
".ffilemanager .fm-maxfiles .fp-btn-add" "css_element" should exists
behat_general::should_exists()

Css matching locator ".ffilemanager .fm-maxfiles .fp-btn-add" not found.