Figure out what the actual sample rate is used in that mode. Some scopes would let you separately configure the memory depth, otherwise they select the slowest that is good enough to fill the screen.
This was the problem with the early DSOs, which reduced the sample rate at large values of time/div, so far that it dropped below the higher frequency components of a PAL analog video waveform, the resultant aliasing making the display unusable.
The first one I encountered couldn't even show a line (horizontal) waveform, whereas the later ones were better, showing a "sort of" accurate line rate signal, but a field rate signal was a total mess.
As the OP's signal isn't near as complex as a video waveform, I'm surprised that a modern DSO can't display it.