[N1QL][ADVISE] ANSI merge query getting wrong advise

When I run this query

ADVISE MERGE INTO `travel-sample` t
USING [

{"id":"21728", "vacancy": true}

,

{"id":"21730", "vacancy": true}

] source
ON meta(t).id = "hotel_" || source.id
WHEN MATCHED THEN
UPDATE SET t.old_vacancy = t.vacancy,
t.vacancy = source.vacancy
RETURNING meta(t).id, t.old_vacancy, t.vacancy;

I get this advise output (which is incorrect):

"recommended_indexes": {
"indexes": [
{
"index_statement": "CREATE INDEX adv_hotel ON `travel-sample`('hotel_')",
"keyspace_alias": "travel-sample_t",
"recommending_rule": "Index keys follow order of predicate types: 9. non-static join predicate."