I concur. There are three. They are also on timers. For instance, if you kill Zorn & Toth, then leave for the night, you'll lose access to the chest between that boss and the twin tanks encounter.
This happened to us last night. So you are positive that it is not a bug? And also, are you saying you won't get the chest from killing the tanks from your example? If so, how does that make sense? We just killed the tanks on Tuesday and got the chest for the tanks and quit for the night. Wednesday, the next chest wasn't there like you said but the next boss gave loot and the next chest between the mine boss and the last boss were there.
Edit - I'm dumb, I just noticed you said only the chest in between the bosses. But still are you sure it's not a bug?