I love this topic!! I may have run a few chests in my time, stumbled over a few on accident (that may have been the grog in or out of the game hehe) but there are several environmental things as mentioned here that will impact chest spawns. The infamous foe pop-up does occur, not in all maps but several, and will at times effect if the chest is spawned prior to the pop up foe spawning or not. Oni is a good example that may not be visible but a chest will already be there before triggering the Oni. Then you have FOW which I run a lot during Pantheon week and that dang pesky spider cave will have a chest pop up behind you 50% or more of the time (always look behind you or hit your find next item button often while traveling through). As Praise mentioned, that Elona Reach chest in the far corner is actually triggered by picking up the shard which in return opens the gate.
There are also times where a chest can spawn literally outside of reach, which you can sometimes see, perfect example is EOTN Kilroy's punchout runs by the wolves, one will often spawn on the other side of the rocks. This makes me wonder if at times a chest can spawn outside of the "path" but not visible cause of where it spawned (just a hunch as there is no way to prove this).
Regarding Pongmei, the typical chest count is 5...….99% of the time. I have had a few times where I got 4 and ran all over the map a 2nd time looking for the one I missed cause stubborn me thinks that is the one with that q8 gold dual modded perfect bladed shield, and most of those times, there is not a 5th to be found. However, this is a statement that not everyone will believe me on, have had debates with others over this, I have seen 6 at Pongmei. This is rare, but it's all about the right amount of space between the chests. Most people think of 5 being the max and will stop, I do the same, but there has been times I have kept going in a very rare occasional, a 6th will be there (so don't rule out but also understand proof is wanted). If this happens again, I will screenshot my chat box with pick counts between the resigns.
With that in mind, not all areas have a set chest count that will be 100% of the time, and the size of the map may or may not always be the deciding factor. A lot of the low level areas even in HM, may only spawn 2 chests, like Jaya Bluffs. On the other hand, one of my favorite chest runs, Ice Dome, that is a relatively tiny map, can spawn 2-4 (yup sometimes 4) chests in a rather small area.
Lastly, the actual topic on hand, I cannot say I have experienced zero chests at Maishang Hills specifically, however I have elsewhere. One example of this that drove me crazy cause it happened often, is chest running at Turais Procession. I have had several occurrences of getting zero chests here even in a row, why, couldn't tell you, but it drove me bonkers!