Help: each update cause the mac to boot from the wrong partition

How to use Apple Hardware Test

1. Disconnect all external devices except keyboard, mouse, display, Ethernet connection (if applicable), and connection to AC power. Apple Hardware Test might display an error message if you don't disconnect all other devices. 
2. Make sure that your Mac is on a hard, flat, stable, well-ventilated work surface.
3. Shut down your Mac.
4. Turn on your Mac, then immediately press and hold the D key on your keyboard. Keep holding the D key until you see the Apple Hardware Test icon:
5. Select your language and click the right arrow. If you aren't using a mouse, you can use the up and down arrows to select a language, then press the Return key.
6. To begin testing, click the Test button, press T, or press Return. Optionally select "Perform extended testing" before beginning the test. This performs a more thorough test that takes longer to complete.
7. When the test is complete, review your test results in the lower-right section of the window.
8. To quit Apple Hardware Test, click Restart or Shut Down at the bottom of the window.

Learn more

• Some Mac computers that shipped with OS X Lion v10.7 or later will start up from AHT over the Internet if your startup disk doesn't contain AHT, or if you hold down Option-D at startup. This requires an Internet connection over Ethernet or Wi-Fi.
• If you're using OS X Lion v10.7 or earlier and you can't get AHT to start, see if you have an OS X installation disc named “Applications Install Disc 2.” Insert the disc in your internal optical drive or external Apple SuperDrive before following the steps above. Or, if you're using a MacBook Air (Late 2010), plug the MacBook Air Software Reinstall thumb drive into your USB port instead.

https://support.apple.com/en-us/HT201257

1. Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later.

Command (⌘)-R Install the latest macOS that was installed on your Mac.

Option-⌘-R Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R Install the macOS that came with your Mac, or the closest version still available.

Release the keys when you see the Apple logo, a spinning globe, or a prompt for a firmware password. When you see the Utilities window, you've started up from macOS Recovery.

>System Preferences>Startup Disk Manager

Select or reselect the Macintosh HD here

How to select a different startup disk - Apple Support

To trouble shoot further you can:

Try a SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc.

Maybe this cache clearing is all it will take to get back on track.

Reboot as normal and test.

Very strange, my best guess is a RAM or Cache problem, have you run the extended Hardware Test?

If you reboot the computer and hold down the option/alt key during the restart, do you get to select what partition you want to boot from?