Minecarts won't turn when traveling at slow speeds

I noticed that minecarts won't turn corners. They get knocked back instead (like when hitting a solid block).

This doesn't seem to happen when the minecart is traveling fast enough though.

The attached screenshot shows two setups. The top one works fine, while the minecart in the bottom one moves back and forth between the two turns, without ever making it to the lower part of the track.