I'm missing something on point number 2.
What do you mean by "return back to sprite editor for a smaller sprite 50x50 pixels" ?
Make a new sprite ? Open the previous sprite and open a 50X50 sprite in it ?
Also what image format ?
I tried with sprite in png to the dimensions you said, icons are still visible.
Also what is your monitor resolution and are your graphic card drivers up to date ? (What is the graphic card too btw ?)
Could you provide a capx and precise steps /how to reproduce the bug please.
So far I've been unable to reproduce it.