Sorry I'm a little late to the party, but this is something I feel should be shared with others. So, after repeatedly playing through chapter 10 about a dozen times this last go round (dozens more in the weeks prior), I noticed a couple things. (1.) Every time I dropped down into the area with the traps and waterfall, the audio would get wonky and flicker between normal and muffled. Every time I opened chest 10 (in front of the waterfall) while the audio was flickering, it didn't do the choir-like chime that the chests normally do. And every time, I would end up with 37/38. After repeated attempts, I tried something different. Before I opened chest 10, I killed the three dudes nearby, and then flipped the switch to slow down the gator-chomper things. As soon as I hit the switch and the gator-chompers slowed down, the audio went back to normal. I then opened chest 10, and to my pleasant surprise, it played the choir-like chime. I proceeded to get the rest of the chests in the level, and ended up with 38/38. (2.) The audio got wonky, yet again, when I got to the fountain near the end, near chests 32 and 33. I was able to find a spot, close to each chest, where the audio would stop flickering. Yet again, once the audio stopped flickering, the chests chimed, and were registered in the official tally, resulting in the above referenced 38/38.
In my experience, the only trouble chests were chests 10, 32, and 33. I did want to point out that, unlike near chest 10, where the audio got wonky EVERY time, the audio didn't flicker every time when I got near chests 32 and 33. That could explain why people sometimes get 35/38, and 37/38 at other times. I don't know if these chests are what are preventing so many people from getting this achievement, but I do know that, after dozens of unsuccessful tries, I was able to get all 38 chests after doing these couple things. If you find yourself in the same predicament I found myself in, try what I did and see if it works for you, as well.
Oh, and I got the achievement without having to replay chapter 10 after all that, so I know it registered, at least it did for me. Also, during the replay mentioned above, I shot the chest on the ledge by the gator-chompers after the chest with the invisible sword (which, I personally had zero problems with. I know it sounds crazy but, no bullshit, it chimed every time I opened it). And, I know for a fact that at least three chests were opened by enemies, both by being shot and meleed.
Also, it might be worth noting that I did not play through all the chapters in any kind of order, at all. I was bouncing around between chapters and difficulty settings while working on a couple other achievments at the same time. (That's how I roll
Hopefully, this might help someone else out down the road. And thank you, CloudedJudgment, for your kick ass treasure chest location guide.