Hi Matthew,
So i'll start that IE11 (11.0.9600) works with the PM new UX if in edge mode. (see screenshot below)
I was able to replicate this problem if the URL is running in enterprise mode, since put simply the emulated version doesn't have support for the code used. (note: confusungly ie11 compatibility settings are different to enterprise mode)
I would start by pulling the location of the enterprise site list and then reviewing the xml in that list for your site.
Location found in the registry editor:
HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Internet Explorer\Main\EnterpriseMode
.
If on premise there is a fix for this, but I think CA will be annoyed if I post it here