I am afraid you need to troubleshoot it, identify which transaation is wrong and probably some correction necessary in an automatic document or so.

To help, I am missing some information:

• Do you have the same issue when running the stock valuation report at the currrent date?
• Is this product set with AVC stock valuation?
• Do you mind sharing a screenshot of the valuation method?

I experienced the same results in 2 cases;

1/when I ticked the accouting date and running the stock valuation in the past. You need to know, that when using the accounting date, if a stock transaction is not posted (error in the stock interface such a missing auto journal), then this record will not be seen by the stock valuation report.
=> So can you check if you still have the same error if not using the accounting date?

2/when I have a product that has its stock valuation modified in the period between the reference date of the stock valuation report and Today.
=> can you check if you have stock issue movements that weren't valued with AVC (STJ.PRINAT_0 <> 5)
It can be as well, that you are using (in the stock valuation method) an alternate value (used when the AVC is nill) that is different from the AVC.