Instinct (original)
1st watch reset the device to factory and changed a couple of things. Disabled 3D Distance and Vert as it's been said to make elevation readings worse instead of better. Going to do the same route tomorrow to see the difference, if any. HR is re-enabled.
2nd watch reset. Altimeter again via DEM this time and changed the Barometer to read from the Altimeter. If that doesn't cure it I'll nuke the watch settings online, reinstall the app and rest the watch itself. If that doesn't work I won't be happy.
3rd watch reset plus deleted watch from account via website and fully deleted app on phone then reinstated.
Nothing has solved the issues.
If I can find the means to do so I'll downgrade the firmware to 10.10 or 11.00 as I had zero issues with those firmware versions. Then I'll just turn off Auto-Update and be done with it.