I have not nail down the exact course of the hiccup, but my army stack is ignoring the road costs of existing roads after I added an outpost...  but I have also been equipping the unit with a scout ring and an amulet of the eagle before I move them around.  It feels like equipping the +sight accessories might be the cause.

• I am playing on the campaign map as the Guardian
• Founded a city by the closest shard East of the Fearful Fog (East of shard)
• Added an Outpost in between them directly SE of the the merchant city, so a new sets of road formed
• I have been equipping both scout ring and an amulet of the eagle before moving the units... at that point they no longer abide by the road movement cost reduction when moving on the old/pre-established roads
• Let me know if you want my save game.