Tier 3 workbench changes to a tier 1 when moving back

This is expected behaviour, as the Engineering Workbench is closer to your position and you've moved outside the Tier 3 Workbench's effective range.

Keep in mind that Facepunch reduced workbench ranges when they introduced the upgrade modules. If you'd like to extend the effective range of your Tier 3 Workbench, you'll need to equip the Workbench Range Upgrade module.

Hope this clears things up!