I purchased this 1/2” piece of walnut (S4S) that looked perfectly fine. I then applied grain filler and after sanding this appeared. At first I thought this had been a glue up, but there’s no evidence of that on the end grain or opposite side.
Here you can see a distinct straight line down the right side of the board...grain does not flow across the line.
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Here’s the opposite side with no noticeable line (which would be down the left side) and grain flows perfectly.
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And here’s the end grain...no noticeable “glue up” line.
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Any ideas what’s going on here?