I ultimately contacted MathWorks support for this question, and found the definitive answer.
It turns out that for my high-resolution monitor, the "ScreenPixelsPerInch" had a default value of 72, which was causing the plot symbols to be rendered in a way that they did not show up on the screen. The dots "existed", but were not rendered visibly on the monitor. (They did show up in the saved/printed output.)
I was able to fix the issue by slightly increasing the value of ScreenPixelsPerInch:
>> set(0,'ScreenPixelsPerInch',76)
I put that line in my startup.m file, and all is well.