I'm no expert, just a fellow owner of a Learning model, but no expert has jumped in to help so I'll share my thoughts and ideas.
1. Perform a Restart of the Nest. If that doesn't fix it, try deleting your schedule (via one of the options under Reset) and then rebuilding it. How to restart or reset a Nest thermostat - Google Nest Help
2. If that doesn't work, reset your Nest password and then log ALL users and devices out. (I don't know how to do either of those things; sorry.) The point of this is to lose any person or program (e.g. IFTTT or similar script) that may have access to your Nest to be able to instigate that daily temperature change. It sounds paranoid to think that somebody is hacking your Nest, but I don't have any other ideas for how the 4pm shift to 89 could be occurring. Though, again, I'm just a user.
3. Perform a full factory Reset. This is kind of drastic and may require a lot of time and trouble. So if you're not ready for that (or if it doesn't get rid of the 4pm gremlin), here's another thought.
4. As a workaround, add another set point to your schedule: 68 at 4pm. Does that prevent the 89 override from happening? If not, move the new 68 set point to 4:15 instead. Whatever the source of the manual override, it ought to stop at the schedule's next set point. Yes, you'll still have 15 min of unwanted heating, but better than going all the way to 89!
