Human Perception of Dog Emotions Is Influenced by Extraneous Factors
tandfonline.com
tandfonline.com
They acknowledge it by referencing that humans mess this up with humans too. A freeze frame of laughing or crying may look the same.
You don't experience a sad or happy person as a still image. People don't have trouble confusing sadness and happiness in people. Because it takes context.
Why would it be any different and not require context for reading emotions in a dog?
Every so often a study goes viral that demonstrates that humans can be fooled about such simple things as whether wine is red or white, whether a dog is happy or sad, whether an accused criminal is innocent or guilty, just by exposing the human making the judgment to some simple extra stimuli.
It will be more interesting when I see a study like this that doesn't find any difference.