Setting Unity project as a WebGl project and the Graphic Api as WebGL 1.0, the outline works on editor but not in build. Also not working with Auto Graphic API in a machine that uses WebGL 1.0 so the problem could be a Shader implementation not compatible with the WebGL 1.0 restrictions.

Unity: 2020.3.16f1
URP: 10.5.1
UnityFX.Outline: Current repository version (1988f71)

Example project: OutlineWebGl1Bug.zip