In Ecclessiastes 6:12 why is Solomon unsure about where man goes after death?

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For example, Solomon keeps saying things like:

For who knows what is good for a person in life, during the few and meaningless days they pass through like a shadow? Who can tell them what will happen under the sun after they are gone?
— Ecclesiastes 6:12 (KJV)

It is perplexing to me that Solomon seems unsure because David knew of the resurrection as well as others in the OT. It also seems to give fodder to those who argue about contradictions in the scripture, and how can we really know what happens after death.

Scripture is clear to me about our death and resurrection, and then the judgement that determines the eternal destiny of believers and unbelievers.

Please help me understand what Solomon is saying in these verses from Ecclesiastes.