Oscilloscope Auto Trigger Mode Unstable Display
Automatic triggering is probably not what you think it means; in many oscilloscopes "auto" mode means that after a pre-defined timeout period if no actual trigger point was reached, the oscilloscope will begin capturing data anyways. There's no reason the frequency this happens has to be aligned at all with the signal you're actually trying to capture, so it shouldn't be a surprise if you get phase error in the resulting super-imposed image (which is why it looks "unstable").

In the first image, the trigger is being properly found so it will correctly periodically trigger and the oscilloscope will tell you it's actually triggering rather than hitting this timeout forced data capture.

In the second image, the triggering circuit never fires (either the trigger level is never met, or if some internal filtering/heuristics is preventing such a small deviation from actually triggering). Instead, you're hitting the auto trigger timeout. There should be a corresponding hint on the display that the trigger has never actually been reached, and instead the auto trigger timeout period has passed.

More information on auto triggering: link