On ice, block edges can become barely visible (unlike what I said in MC-128063, where they suddenly and clearly completely disappear). When that's at least possible to notice something.
In short, it seems to occur when these ice blocks are above water.

For example, in screenshot #1, edges show properly, but in screenshot #2 it becomes quite faint already. There's no water under that block though, and according to my tests, here it seems to be because adjacent ice blocks create some kind of shadow to one another, making the dark line merge in it (still should probably be more visible, though).
However, I'm pretty sure it's supposed to be much more visible in a situation such as the one in screenshot #3, where we... really can't see anything. If looking at this block from the side, it becomes possible to notice something, but that's pretty hard (see screenshot #4).

And screenshot #5 sums up a little all that : The part of the lines with "normal blocks" behind show properly, but when they start to cross water or ice, they become barely visible.
.

EDIT : About the first screenshot, the block isn't directly on the ground but it doesn't matter (the name has "on ground" because what's direclty underneath, apart from ice, is ground, not water).