Whenever I want to go back on a novel and reread a part I enjoyed or forgot about for context, it messes up my place in the novel. For example in I'm Really a Superstar chapter 850 they were talking about Yao and Zhangs crosstalk. I would like to go read it, but if I go back to 470~ then I will lose my place. The same thing happens if I have it open on my phone and laptop. If I stop at chapter 800 on my mobile then read to chapter 850 on my laptop when I come back to my mobile it resets back to 800.
How to Reread favorite parts.
Open the site in another browser in which you haven't logged in and read there so that your latest progress on chapter won't be synced.
Gor just right click the chapter and open in private window
Opening in another browser or private window where you're not logged in isn't the nicest solution imo, because its both unnecessary and the chapter you want to read may be behind a paywall.
In your first example (going back to read a previous chapter to reread a part). Leave the tab where you're reading open to your latest chapter (i.e. 850) and open a new tab to read the older chapter in (i.e. 470~). Once you're done reading the old chapter, close that tab and continue in the first tab. You might need to change the chapter to get it to register though (I can't test atm due to the site problems going on). So just scroll up to previous chapter and back to latest chapter real quick. i.e. scroll up to 849 and back to 850. The left and right arrow keys are hotkeys to change chapters quickly.
In your second example of reading on two different devices. Make sure you bring up the story either through your library or the continue reading link of the story's main page. Leaving the page open to the last page you've read may be the most convenient but will result in the issue you mentioned.
It might be nice if they implemented something that marked the latest chapter you've read but that has a few problems such as, what if you miss-click on a later chapter or you go and pay the ss for a chunk of chapters. Then that latest chapter mark will be wrong and useless, causing readers confusion and annoyance. I'm not sure if there is a perfect solution to that since that latest chapter mark would either have to be automatic, and so can be false if reader accesses a later chapter for X reason, or manual, which you can already do with browser bookmarks and similar things and has the issue of the reader having to remember to use it (whether that's an issue or not is different per user). The nicest might be a manual bookmark that will carry over between devices.