I have seen other cases where rules start to fail due to the hub resources getting pushed to the limits. Especially found on rules with waits in them, the rule will start and then never finish. Typically this would be caused from custom apps or drivers, especially ones making a lot of IP connections. It is compounded if those drivers or apps cannot connect to the destination and keep trying over and over again.
Another possibility is that you are having a rapid decline in memory which has been happening randomly to people on 2.3.6 firmware. A database backup and restore typically fixes it, so I would suggest just trying that as it cannot really hurt anything. It is explained in this pinned post: ‼ READ FIRST - Before Posting in Get Help as well as a lot of other info and tips.
