Steps to reproduce the issue

Itemid was supposed to be inherited from the current menu item - this is how it's always been as far as I remember (1.5.0) and what the SEF docs say.

• SEF turned off (doesn't matter, issue occurs regardless of SEF but it's easier to debug)
• Create an Articles (com_content) menu item - doesn't matter which type
• Open /components/com_content/content.php
• Replace it with the code:

var_dump(JRoute::_('index.php?option=com_content&view=test'));
var_dump(JRoute::_('index.php?view=test'));
die;

• Access the menu item.

Expected result

string(63) "/j384/index.php?option=com_content&view=test&Itemid=121" string(63) "/j384/index.php?view=test&option=com_content&Itemid=121"

Actual result

string(48) "/j384/index.php?option=com_content&view=test" string(63) "/j384/index.php?view=test&option=com_content&Itemid=121"

System information (as much as possible)

3.8.4

Additional comments

This is a regression from #19099
If you replace libraries/src/Router/SiteRouter.php with the one from 3.8.3 everything works as it should.
If there's no option= in the URL, then the Itemid gets inherited correctly, but I don't think developers should have to rewrite all URLs because of this (what I consider to be...) B/C change.