I do this all over my application. The key is to set your condition to is in (or was it contains? I always get confused. I’ll assume is in from here onwards.)
Instead of key = Multi Dropdown’s value.
Why? When you put =it expects the expression to evaluate to a single item (be it text, number, other data type, etc.). A Multi Dropdown will always evaluate to a list. Even if you select only 1 item, it’s value will be a list with only 1 item.
You might want to check the ignore empty constraints in your search so that the repeating group displays something even if you select nothing in the Multi Dropdown.
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