Looking at an any block puts a black outline around it.
Looking at ice blocks, sometimes puts a black outline around it, and sometimes the outline is the background behind it.
I have attached a few images, one has a black outline, one has background as outline, and one is a bit between them.
It is most noticeable when the background is white
Environment
Vanilla 1.20 Server, no plugins.
Rendering issues like these are only valid if they occur in fabulous graphics; this does not. Fabulous is specifically made to fix these kinds of issues.
Duplicate of MC-259798.