It is a bug.  I've seen it before.  For some reason, the system will sometimes refuse to path across a river, instead going completely around it, even though it may take many more turns to do so.  This problem has been around since before I started playing in January.  It's not a frequent problem (although that might be more because I've learned to not use auto-pathing because it keeps getting me killed), but it does happen.

The path is determined by the path that a Pioneer would take instead of what a Scout would take, which is what we want.

In no case is it better to spend 4 turns going around a river than spending 2 turns to cross it and reach the same tile (which is the situation in the screenshot).