This is a known bug with Yongnuo flashes (notably the YN-685's predecessors, the YN-568EX and YN-568EX II). TTL tends to be inaccurate and will underexpose unless you switch the metering mode from Evaluative to Average (see: this DPReview discussion, where one person claims Yongnuo support themselves suggested switching to Average metering).

This is, after all, a $100 reverse-engineer flash of cheap Chinese manufacture. While it may have some feature parity with Canon's flashes, it will always lag behind on copy and component consistency, backwards/future compatibility, warranty service, and resale value. They're great for hobbyist usage, less so for professional usage. While their QA has gotten noticeably better since the days of the notorious Strobist review of the YN-560, the low-low price has gotta come from somewhere. And if it's a new model and you're an early adopter, you may still be an inadvertent beta tester. Yongnuo has many quirks, and the fact that aficionados can tell you where to find manufacture date codes and which dates align with which silent updates is only one sign of this.

You may want to look into Godox's Wistro (bare bulb) and Ving (speedlight) flashes, Phottix's Mitros+ speedlights, or (if you want to go manual-only speedlight) LumoPro's LP180R for pro hardiness and reliability—not to mention better trigger interoperability (Yongnuo's got three separate triggering systems—none of which work together) if you still want to go third party vs. OEM. The Flash Havoc blog is a good source for what's out there.

Most folks purchase a YN-685 simply to have a cheap YN-622C off-camera slave. I love my Yonguo gear for off-camera, but for on-camera bouncing, I'll grab my 580EXII first, every time.