I have seen this as well. It only happens on load, and only if the bot was using the machining table during the actual "save" process.

for me it seems to happen when a bot dies and you try to repair it. i tested it a bit.

The same is happening for me and i can confirm its not just on loading saves, it happens for me mid-game. the last two times i noticed after a mechanoid attack when i wanted to diss-assemble my bench was gone and a robot in its place but i wasnt missing any of my robots, this one was new and un-usable as when i tried installing it it had no base plate, he just was a sad little lump on the ground. I was able to Dis-assemble it for robot components though.