How to lock network operator or to never go on roaming, or get out of roaming as soon as possible
It seems that Android M Marshmallow does fix this bug.

I am now running Android 6.0.1, CyanogenMod version 13, and the operator does not revert back to automatic-selection anymore after reset.

The exact CyanogemMod version I am running is: 13.0-ZNH2K-20160430-STABLE-Sultan-bacon.