It's a bug found in version 4.1.1 and 4.1.2. The problem doesn't exist in version 4.0.1. My printer configuration is Anycubic i3 Mega.
The missing infill layer in the attached file is at the height of 15.80mm. Some other models could have more than one missing infill layer, these problematic layers will shift up or down when the value of Infill Outline Overlap changed.
Why one single outline shell? In order to get nice printouts of small and thin lithophanes, a single outline shell and 100% infill seems to be the only way to get the job done. Setting the value of Outline Shell to 2 gives an ugly output.
Using version 4.0.1 is a workaround. But the slicing result in version 4.1.2 is way better than that in version 4.0.1 (4.1.2 generates much more smooth layer lines). I've spent days to tweak all related parameters in 4.0.1 with different models and printed like over a hundred testing samples. None of them was as good as the printouts generated from 4.1.2, except there is no missing infill layers.
Please fix the bug, thank you.