Action to write value summed from children tests fine but return zero

[OptimiX_XcrY]

Hello,

In my attempt to optimize my apps , today I replaced some spreasheet formula with an appsheet action button, to write in a parent column the sum of a child table column (set values in this row) , something like

[ParentColumnSum]=sum([RefColToChild][ChildQuantity])

When testing the formula, results were perfect in testing page.
When using the action, it wrote 0 in my spreasheet.

I rewrote the action using select instead of child referencing

[ParentColumnSum]=sum(select(etc…)) , and it works just fine.

There were no slices, no security filters, so my question/curiosity is why the first version of action tested correctly but wrote zero in spreasheet.

Thank you for helping me understand