@bradk I don't know much (and less about shaders). The funny thing is there isn't even a GameObject at that location, so I don't know what is failing to be shaded. Also, this doesn't happen when running the same WebGL build on a different OS.
Error messages in the browser console support your hypothesis, but I don't know what to do about them.
Note: Creation of internal variant of shader 'Hidden/PostProcessing/FinalPass' failed.
q3.framework.js.gz:10:34907
ERROR: 0:2: 'GL_AMD_vertex_shader_layer' : extension is not supported
ERROR: 0:20: 'gl_Layer' : undeclared identifier
ERROR: 0:20: 'assign' : l-value required (can't modify a const)
ERROR: 0:20: 'assign' : cannot convert from 'highp int' to 'const highp float'
q3.framework.js.gz:10:34907
0(285) : error C7532: global function uintBitsToFloat requires "#version 330" or later
0(285) : error C0000: ... or #extension GL_ARB_shader_bit_encoding : enable
0(285) : error C0000: ... or #extension GL_ARB_gpu_shader5 : enable