I am attempting to create a glow effect around a line. I created the line, then duplicated it and used the blur slider in the fill and stroke tab to blur it.

When I blur a line that is perfectly horizontal or vertical, it disappears when any amount of blur is applied. If the line is tilted, even by 0.001°, it blurs correctly.

I am on Windows 11, running Inkscape 1.2.1 (9c6d41e410, 2022-07-14) installed from the .msi installer.

I'd try using Path>Stroke to path, before the the blur.