Apparently, that additional inline setting was meant specifically for IE 8 and lower when detected but they're not using the corresponding ms- prefixed attribute for writing-mode at the same time for IE8 (or IE9) so I don't know what is going on there exactly. The specification by w3.org differs from MS's approach as well and I believe some of those 'top-bottom, left-right' writing-mode values applied only to objects that are .svg images (if I'm reading that right) and may have been deprecated at some point to boot.
Obviously, something is crossed with either the scripting for IE version detection and/or the appropriate attribute to use under each version of IE. Whatever the case may be, imo any fix 'should be' vetted against IE 8, 9, 10 & 11 independent of whatever claim any documentation makes as being valid before that patch is rolled-out. (Fyi - Does not happen with Microsoft Edge).
Originally reported in en.wp's Village Pump