For some reason, assign uses the embedded layout (you can see a discussion around that in MDL-64948). If the assignment is used in a real embedded situation, such as when published over LTI, this specific line of CSS causes the embedded assignment to hide content. There's no scroll, so students can't see the submit button.
https://github.com/moodle/moodle/blob/9340b2a18e23836651e6344553269f164195e444/mod/assign/styles.css#L450

Either this should be "overflow:auto" (need to check why this was added in MDL-52954) or this style should be made specific to the page which it's supposed to impact (the grader? I'm not certain).

Right now, to replicate this:

1. Set up two Moodle sites - ltiplatform and ltitool
2. In the tool site, publish an assignment
3. In the platform site, consume the assignment (create preconfigured tool and use it in a course - embedded view mode of course)
4. In the platform, launch the assignment over LTI as a student
   Expected: You can see the submission button
   Actual: In a full screen view (1920 x 1080 was used here) you can't see the submission button or scroll to it.

Workaround: Reduce the viewport width until the breakpoint 767px is hit and the overflow will change to auto and you'll be able to scroll down