One thing to be mindful of is that mystery chest locations will change if the 24hr clock rolls over to the next hour.

So say you're in an active Helltide (e.g. with 40mins left of Helltide), looking at the user updated map, and it's currently 13:55. At 14:00 the mystery chests will respawn to one of the other potential locations (but can also remain in the same location). So if you're checking it just after the hour, the location may not yet be accurate.

As an aside, that means if the Helltide timer crosses over the hour, you have the potential to open 4x chests instead of 2x!