Quoting literal causes no results to return - sql

Question

I have the following query
SELECT v.id
FROM visitors v
LEFT JOIN trackings t on v.id = t.visitor_id
WHERE v.app_id = 'xxx'
AND (field = quote_ident('app_name')
AND string_value ILIKE quote_nullable('test_app'))
Problem is that the query fails to return any results when I use quote_nullable on the string_value. It only works with quote_ident, but that is not the appropriate function as far as I understand here.
Example in pseudocode of how I am building the query serverside (using pattern matching on the filter (starts_with, ends_with etc.).
filter = user_params["filter"]
field = user_params["field"]
value = user_params["value"]
...
"starts_with" -> "(field = quote_ident('#{field}') AND string_value ILIKE '#{value}' || '%')"
"ends_with" -> "(field = quote_ident('#{field}') AND string_value ILIKE '%' || '#{value}')"
"contains" -> "(field = quote_ident('#{field}') AND string_value ILIKE '%' || '#{value}' || '%')"
"does_not_contain" -> "(field = quote_ident('#{field}') AND string_value NOT ILIKE '%' || '#{value}' || '%')"
Later in the DB, I invoke a stored procedure that looks something like
filter_length := COALESCE(array_length(filters, 1), 0);
filterstring := array_to_string(filters, ' OR ');
IF filterstring = '' or filterstring is null THEN
filterstring := '1=1';
END IF;
sql := format('
SELECT v.id FROM visitors v
LEFT JOIN trackings t on v.id = t.visitor_id
WHERE v.app_id = app_id and (%s)
group by v.id
having case when %s > 0 then count(v.id) = %s else true end
', filterstring, custom_filter_length, custom_filter_length);
RETURN QUERY EXECUTE sql;
Where filters is an array of conditions (the ones I built earlier)
Any ideas?