I cannot tell you the reason but I can tell you how to fix it: add unicode-bidi: bidi-override;. See more about it
<div style="direction: rtl; unicode-bidi: bidi-override;">
[x]y
</div>The description
The unicode-bidi property is used together with the direction property to set or return whether the text should be overridden to support multiple languages in the same document.
is not clear enough to explain the behaviour. However, it works.
EDIT
The MDN article brings some light here, bidi-override actually disables the browser standard smart behaviour and everything works as is / as expected.