I found that same pattern when I used very low gains, see the end of that post. It looks like a moiré pattern interaction between the sampling of the scanning and the low signal level at low gains. It went away for me at even slight gains since the signal dominated. Either way it’s an artifact of the sampling and rendering in that form when signal is very low.
If you’re seeing it when you expect to be seeing signal it implies that the scanning software isn’t seeing the signal data. You may also find that certain gains just have the right ‘beat’ interaction with the sampling and render to produce them. You should still see signal clearly when there is some.