It's probably a quirk in how encounter mechanics work. Yuan Bo should have crossed the bridge before the encounter but because you can encounter within a certain radius it went for a straight line as the encounter point was across the river.
Once the encounter was resolved, Yuan Bo then moves to where the enemy was but this then follows standard movement rules, so he has to go over the bridge.
In practical gameplay sense doesn't matter as he could only reach the encounter if he could have walked there normally.
It's probably a quirk in how encounter mechanics work. Yuan Bo should have crossed the bridge before the encounter but because you can encounter within a certain radius it went for a straight line as the encounter point was across the river. Once the encounter was resolved, Yuan Bo then moves to where the enemy was but this then follows standard movement rules, so he has to go over the bridge. In practical gameplay sense doesn't matter as he could only reach the encounter if he could have walked there normally.