Thanks. The width of those elements is 0.4mm in the CAD program. But I can understand how dimensions that display as 0.4 may actually be ever so slightly smaller as a result of limitations of internal representations and finite precision math. (Some numbers can be exactly represented in a fixed number of digits base ten are repeating decimals in other number bases, and vice-versa.) The elements appearing/disappearing in different copies on the same build plate/same settings are probably the result of placement transformations throwing tiny difference between the actual width and 0.4 mm back and forth between below and above 0.4 mm. On seeing your comment, I did some experimenting with line width and the Outer Wall Inset parameter. When I changed the Line Width in Cura to 0.39 mm, the Outer Wall Inset setting went to 0.005, and I still had some missing elements. But changing the Outer Wall Inset to 0.0 (while leaving line width at 0.39) caused them to show up. They also showed up with the combination of Line Width = 0.4mm and Outer Wall Inset = -0.01.
One computer language from my youth, APL, had a comparison tolerance setting that controlled how close (in relative terms) two numbers had to be to be considered equal - useful for decisions in calculations like these. Cura might want to use some similar logic.
When I further increased the number of copies (with the original Line Width and Outer Wall Inset settings) some of the similar elements were missing in some of those copies as well, further refuting my initial thought that it had to do with first vs successive copies of the model.
Anyway, thanks again. Glad I can now print the model with the additional elements present. They are significantly helping the success rate printing small-footprint items in ABS.