Dock Preference on animation keep resetting

You could try trashing the file containing the Dock preferences, and set them afresh.

Sometimes plist files can get subtly corrupted, and that may cause erratic behavior.

In Finder, press Command-Shift-G and paste

~/Library/Preferences/com.apple.dock.plist

Drag it to the trash and then logout and login.

Set the Dock preferences again (they will have reverted to default) and see if they now stick.

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.