Just an FYI for anyone running into this: Until it's fixed, a good way to catch this early (thus preventing too much lost progress as far as chapter tracking goes) is to check the icon for your account dropdown in the top right corner. When the issue happens, it'll show the black/dark grey guest user icon instead of the MD cat icon.
If you're on the series info page instead of the reader, a couple of other symptoms to look out for are the read/not read eye symbols next to the chapter names being missing (it'll still show the little blue highlight on unread chapters, it's just that the eye icons are missing), and sometimes you'll get some sort of "lost communication with the reCaptcha server" text (I can't remember the exact text, and I'm not about to wait for the issue to occur so I can copy it) that goes across the bottom of the info area (roughly where the Alternative Titles section is). Said text is a single line that overruns the left side of the page (cutting of most of the beginning of the text), a clear indication that it doesn't really have any sort of container or formatting applied to it.
If you see any of these, then the issue has almost certainly occurred and you should refresh the page to fix it. Refreshing the page is usually enough, though sometimes in my experience it asks you to log in again (I've never run into it needing the password to be manually re-entered). Of note, it specifically has to be a refresh. Navigating to other pages does not resolve the issue.
Also of note is that, once this happens to one of your tabs, it's going to happen to all the tabs you have open since it appears it's session based and not just tab based.