Steps to reproduce the issue

You can use a default install with Isis as administrator template. I just tried it on the 90 Minute test drive available on Joomla Homepage.

Open/Login to Administration. Resize your browser window so that the mobile menu appears.

Open the mobile menu. All main menu entries will be shown.
Open the sub-menu components. Only the first entries will be shown.

Reason for this is a fixed height: div class="nav-collapse in collapse" style="height: 256px;"

Now the interesting part:
Close the menu and open it again. Now the menu is usable, the above line has changed to:
div class="nav-collapse in collapse" style="height: auto;"
To reset to error constellation just reload the page.

So, I guess there is a little bug that the code responsible for setting auto is not called on first load.

Expected result

All sub-menu items visible.

Actual result

Only items visible which fit in the first 256px of menu.

System information (as much as possible)

3.4.0

Additional comments