Reproduction Steps
Red Triangles indicate miners
Yellow Squares for smelters
Blue Circles for manufacturesÂ
Black lines for conveyors
I've run through these steps twice now with the same result, hopefully this data can help out some way or another
Step 1 - Load Save Before
Map should be like below - Main base near the radar dish, you start on the cliff near the stone mine.
Step 2 - Create additional miners (Red Triangles) on the stone, copper, quartz and iron spots up the top of the map.
Step 3 - Create Smelters at iron and quartz spots up top (yellow squares)
Step 4 - Build the Manufacturing machines (blue dots) at the top iron mine and wire up all the conveyors for the iron mine
Step 5 - Head back to the top quartz mine and wire up the conveyors.
Step 6 - Walk back to home base to find middle smelter at the iron mine missing (purple cross)
Oh also, the dot on the map for the copper mine behind the satellite dish is incorrectly positioned ;)
UPDATE
Just done another run as vanilla as possible. only focused on building machines, nothing else, as little quests as possible, only slept once in a prebuilt bed, all mining done manually, no power whatsoever.
Built 20 sets of machines in one area without an issue, then went afar and built just one set of machines, came back to one machine missing out of the original 20. All before winter, so snow isn't the cause either.
The saves below are as follows;
"Save at point of Automation unlocked" - exactly that, just went hard at unlocking the Automation tech.
"Save after 10 machines wired up" - built 10 sets of furnace / manufacturing with only conveyors between them, all at the northern iron mine.
"Save after 20 sets" - same as above but with 20 sets.
"Machine Gone" - same as above but built one set including conveyor over near the satellite dish, came back to the iron mine and one of the machines was gone.
It appears to be distance based, I am yet to see one disappear in my immediate vicinity.