Let’s say you are reading this very valuable post of mine here. And you can tell even after reading just a word or two that you want to mark it unread right now without further ado.
Down at the bottom of the screen we see
“Share Bookmark Flag Mark unread Reply”
OK, we click “Mark unread”, and yes, it indeed marks it as unread.
But then we get sent to the site’s homepage.
Somehow it is assumed we want to leave the page and go somewhere else.
To be fair, I don’t know how else you would keep the post unread, given that merely looking at a post will end up marking it as read, so the only way to absolutely prevent that from re-occurring is to navigate away.
Myself? I typically just bookmark with a short duration. That way later I am prompted by the specific post I want to read later. 
If you want to not read or be in the post right now (and are hence marking it unread) this sounds like a fair assumption to me.
I guess for another example, when I mark a message as unread on Discord, it doesn’t immediately whisk me away – it moves the ‘read’ indicator back up and maintains it after I leave to go to another channel. It will stay unread until I come back to the channel and read the messages.
Maybe it’s a bit different with topics though – I don’t use this feature very much at all in Discourse, so I can’t really say what expectations / preferences would be most helpful for me.
Well Discourse must have an overly aggressive “read” detector.
It’s like I dare not have a look at the last page of a library book, else it will then fly out the window and back to the library.
Discourse should mark the item as read when navigating there. But that’s it.
No. Discourse thinks you will close that book, go home, and start reading again from beginning. If you don`t read the whole book, aka. a topic, it is still unread on /latest — if there is more than OP, and that situation you will close, again, that book and go home to re-read it, aka. you mark topic as un-read.