On my Mini 1 the print bed rises and lowers as it moves back and forth during printing. It is not much but visible if you watch closely. I thought it may have been a bent guide rail so I replaced them but still have the problem. This up and down motion is only on the left side of the print bed and it seems to be at the same place every time. The wire chain does not seem to be causing the problem either.
See the attached videos.