The bug
The edge texture is mapped onto the top and bottom of glass panes in such a way that it "repeats" within the same block space, which is unexpected.
How to reproduce
Compare the texture on the top of a glass pane to the texture on the side (when connected to a block you can see through). The top should match the side, but does not.
How to fix
The resource pack attached to MC-267315 fixes this issue entirely.
Linked issues
discovered while testing
is duplicated by
Attachments
Comments

Can confirm still in 1.21.4