This issue only affects Java Edition 1.16 and above. I found it when making a throwback texture pack.

Using an identical minecraft.png file (located at /assets/minecraft/textures/gui/title/minecraft.png) testing against both versions 1.15.2 and 1.16.1, you can see there is a visible difference in the outlines as rendered in both clients.

I'm including a copy the texture pack used below for external testers wanting to verify the bug using the same texture pack for each version tested.