g:availability showing as “instock” and not as “in_stock”

Hello, it seems that our Google Shopping Feed is sending the value for the g:availability field as “instock” and not “in_stock” as per google’s requirements. I have checked the one other question on this forum regarding the subject to which it was recommended to update to the latest version in order to fix this issue, but we are using version 13.0.8 and we still have this problem.
Also, can please advise what value to use for this field.

Thanks in advance.

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