I’ve been experiencing a minor version of this for some time, but it seems to be getting worse.
Chrome Version 76.0.3809.100 (Official Build) (64-bit)
When reading, randomly stories will start to get marked as read. This can be one -> many stories. It can happen to blocks of stories at a time. It can also continue to mark random (non-sequential) stories read without any input entry from me (no mouse movement or keyboard strikes). This has happened in individual feeds as well as folders and all stories.
This has been a frustrating issue. I usually end up chasing the stories and marking them unread, while stories are being marked read at random, then closing the browser tab quickly to not lose any more stories.
Let me know if I can offer any more details that might help track down the issue.