Hi,
If you click on the Element preview, you will see that 11 elements are matched, which indicates, that the selector has to be made more specific.
This should give you the desired result:
{"_id":"dropdown_test","startUrl":["https://www.morningstar.se/se/screener/fund.aspx#?filtersSelectedValue={}&sortField=name&sortOrder=asc"],"selectors":[{"clickActionType":"real","clickElementSelector":"[id=\"ec-screener-input-page-size-select\"] option:nth-of-type(3)","clickElementUniquenessType":"uniqueText","clickType":"clickOnce","delay":2000,"discardInitialElements":"do-not-discard","id":"50_rows","multiple":false,"parentSelectors":["_root"],"selector":"#ec-screener-input-page-size-select","type":"SelectorElementClick"}]}