I'm not sure if I would consider this a bug, since the vanilla redstone dust textures and model are specifically created to work together (no overlapping pixels, not connected at rising corners) and people who want to aim for a connected look are now able update the models as well.
Can confirm for 1.14 Pre-3, 4 and 5 on Antergos.
Happens in the latest snapshot (19w09a).
Still happens in the latest snapshot (19w09a).
If I had to guess what causes this problem, I would say it's because the formula for calculating the note colors sometimes returns negative values.
Maybe the particle rendering code change in 1.8.2 made it no longer like negative colors.
In that case the easiest way to fix it would probably be changing the color formula in the particle class.
It would be really nice if this was fixed before 1.14 comes out with the new instruments. Cheers!
This is still an issue in version 1.12 on GNOME 3.
As well the the reported behavior, the window reacts strangely when attempting to move it to the second monitor using keybinds. As well as that, pressing super to open the current activities causes the window to disappear but still receive mouse events. The only way to regain mouse control is to use alt-tab and un-fullscreen Minecraft.
Tested on Antergos, GNOME 3.24, OpenJDK 8u131-1
Confirmed for 1.14
This is actually a pretty serious bug, since it prevents the player from setting any NBT strings which need to contain newlines without using an external tool to edit the world.
The only reason it doesn't come up very often is because most in-game text displaying things support JSON.