Using "start previewing node" in a post process material does not work. The preview will only show the final result that is connected to the material attributes.
Working as expected in 4.18 CL# 3832480
Found in 4.19 CL# 4033788, 4.20 CL# 4369336, 4.21 CL# 4408769
This is a regression.
The preview will stay black
The preview would show red like the "Constant 3Vector" is set
Head over to the existingAnswerHub thread and let us know what's up.