The bug

Glass pane items are rendered as 2D items. As a result of this, their appearence does not accurately reflect how they are when placed, as several parts of the item appear to be 3D cubes whereas their placed form applies this texture as a 2D cuboid face. While this could be said of many items such as grass, I consider glass panes to be a more significant issue since they appear as a cuboid both in item form and when placed, exaggerating this discrepancy beyond reasonable expectations.

How to reproduce

1. Hold a glass pane
2. Notice how the streaks appear to be made of 3D cubes
3. Place a glass pane
4. Notice how the streaks are 2D as expected

Expected results

The item form would not be extruded in this manner, and there would therefore be no such cubic streaks.

Actual results

As the glass pane item is handled as 2D, these streaks do end up becoming cubic.

How to fix

Attached to this ticket is a resource pack which fixes this issue completely by creating a new, dedicated 3D model for glass pane items. Alongside eliminating this issue, it also brings feature parity between Java Edition and Bedrock Edition due to the latter displaying panes like this already, brings internal parity between glass panes and similar blocks such as fences and walls, and also fixes MC-36498.

Mojang have my full permission to (and are strongly encouraged to) use the assets included in the resource pack to fix these issues.