I can confirm that this is a issue, and a big one if you ask me.
The issue is with the new 1912 version (build 12325.20298). On version 1910 it wasn't a problem and as @gigidiv has confirmed, it was working on 1911 also.
The workarounds I have found until now, isn't 100% useable as Outlook either ****.. it up totally compaired to a normal browser (Chrome, Firefox, Edge, IE), or the line isn't 100% in width
Our company branding says, that the horizontal line should be 100% of the window, and I haven't figured out how to solved that in Outlook yet. Hopefully MS will fix this over the weekend????? ;-)