I am afraid that this is a known bug.
This was affecting staff at the OU, so I looked into it about 6 months ago. Unfortunately, I could not find any way to fix this without completely rewriting the code, which did not make sense just for an old browser. Also, the current code does work on IE9 and later.
Note that Microsoft has not supported IE8 for ages either.
If memory serves, the change that 'broke' this was MDL-40313.
(And, if anyone wonders what we did about this at the OU, a few weeks ago we put a bit in our theme, so that for OU staff, when they are on a page that we know does not work in IE8, we display a message on-screen suggesting that they upgrade their browser. As far as we are aware, everything students see still works in IE8. If anyone cares, this is our test for which pages to show the warning on:
// Course view page when user is editing.
if ((preg_match('~^course-view~', $PAGE->pagetype) && $PAGE->user_is_editing()) ||
preg_match('~^(mod-.*-mod|mod-quiz-edit|question-edit|question-type.*)$~', $PAGE->pagetype))) {