Does the desired rendering happening in debug mode or in production mode? If IE has trouble parsing minified CSS, then the correct rendering would occur in debug mode, and in production mode the rendering would be broken.

Depending on which is the case, this bug is a lot more important as we'd be actively breaking certain CSS rules for IE in production. And with the upcoming plans to use source maps instead of unminified responses in debug mode, we'd be breaking debug mode, too.

In T52830#955117, @Krinkle wrote:

Does the desired rendering happening in debug mode or in production mode?

According to T52516: Vector: IE9 renders the sidebar with extra separators, the correct rendering happens in debug mode.

("IE seems not to see this when the code is minified. However, running with "?debug=true" causes proper rendering.")

On French wikipédia, still with IE9 there is different behavior on debug mode and on normal mode.
The major problems are the edit button (in wiki mode: "Modifier") doesn't display neither the portal parts nor the categories.
It was described the 7th august on Le Bistro (the French village pump) first in this thread
https://fr.wikipedia.org/wiki/Wikip%C3%A9dia:Le_Bistro/7_ao%C3%BBt_2015#Les_portails_et_les_cat.C3.A9gories_n.27apparaissent_plus_.21

@xcombelle That might have been a different issue that we have tracked as T108727. Can you verify that it's fixed now?