Hey @Max_Pauwels,
First, no need for FormulaMap here. All you need is:
Inventory.Filter(
thisRow.[SKU's].Split(", ").Contains(SKU)
)
Secondly, please note that your SKU's column doesn’t contain a list of SKUs, but a single string with SKU’s joined together with a comma and a space. Hence the Split(", ") there. If you intend to keep entering multiple SKUs like that, I advise you to make a separate column where you have your SKUs split into separate list items (thisRow.[SKU's].Split(", ")), then use this new column in the formula above without the Split step.
Don’t forget to change your “Shopify inventory” column type to Lookup from Inventory table and turn on the “Multiple items” toggle. Please be vary that even if the result is only one item, it is still wrapped in a list