Does it help if you remove the spaces from the &fields configuration? It seems there’s an extra space where there shouldn’t be in the query, maybe that’s where it’s coming from.
[[getRelated?
&parents=`8`
&limit=`6`
&fields=`tv.Product_Subject:8,description:4,introtext:1`
&returnFields=`pagetitle`
&returnTVs=`Product_Price, Product_Image`
&tplOuter=`getRelated-Products-Outer-Tpl`
&tplRow=`getRelated-Products-Row-Tpl`
&debug=`1`
]]