It was because there was no actual data with that meta field in the database yet. I guess it is a new field, that's why it has no data yet. Because the search looks for fields in the post meta table, it was not able to find it, as it was not used yet.
To solve it, I have saved the latest event you had on the list on the event editor, that way the new field has been saved for that event, so there is at least that one record in the post meta talbe about it – and now the field can be found in the search settings as well.Best,
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