Horizontal faces not generated in Cura
Cura has several settings that can cause it to erroneously omit small top/bottom surface &quot;skins&quot;. Look for preshrink (&quot;Skin Removal Width&quot;), expansion (&quot;Skin Expand Distance&quot;), and particularly the limits on when expansion takes place (&quot;Maximum Skin Angle/Minimum Skin Width for Expansion&quot;).
Normally Cura shrinks then expands skins by the same amount, proportional to line width, to avoid generating skin (which is slow to print and harms layer adhesion when mixed with walls) in places where the walls will already cover it. But the recently added max-angle/min-width settings cause the expansion to be skipped in certain places, leaving just the shrinking, and thereby serious gaps in the surface. This feature is just misguided and should be disabled by setting the min width to 0.
If you still have problems after that, you can try lowering both the shrink and expand, possibly even all the way to 0, but this will harm print quality and shouldn't be necessary.