Locking rows in DB using R2DBC PostgreSQL and Webflux

I have an issue when trying to concurrently select rows in postgesql database using R2DBC driver.

I have method that should create entity with order number based on total quantity of these entites in specified zone.

val operator = TransactionalOperator.create(
            transactionManager,
            DefaultTransactionDefinition().apply {
                isolationLevel = TransactionDefinition.ISOLATION_REPEATABLE_READ
                propagationBehavior = TransactionDefinition.PROPAGATION_REQUIRES_NEW
            }
        )
operator.executeAndAwait {
            val existingEntities = repository.findByZoneWithLock(id)
             
            val entity = TestEntity()
            entity.order = existingEntities.size() + 1
            repository.save(entity)
          }

Repository from the service with locking query:

interface EntityRepository<TestEntity> : CoroutineCrudRepository<TestEntity, String> {

       @Query("Select * from TestEntity where zone = :zone for update")
       fun findByIdWithLock(zone: String): Flow<TestEntity>

}

When trying to execute 2 concurrent requests that invoke my method I am expecting next behaviour: 0) We have 1 precreated entity with order 1 in DB

1. One of Transactions gets 1 entity and locks existing entities
2. Second Transaction will be waiting for the releasing the lock of first transaction
3. First transaction create new entity, sets the order to 2 (based on size of existing entities), save it and finally commit
4. Second Transaction gets updated amount of entities from the DB with size = 2
5. Second Transaction creates the entity with the order 3 and commit

This is common behavior like if we execute this actions in DB directly with SQL.

However, in the result I can see that I have 3 rows, one with order = 1 (precreated) and other both with order = 2. For some reason on the 2 second step Second Transaction didn't wait for the lock and got 1 precreated row as well. I've tried to change isolation levels, but it didn't help (Serializable just throws exeption in Second Transaction). Also I've tried to change TransactionOperator to @Transactional and replace @Query with "select for update" with @Lock(LockMode.PESSIMISTIC_WRITE), same result

What can be the reason for such behavior? I am new to R2DBC and couroutine repository, but in common non-reactive JDBC it works as expected. Maybe I don't condider some driver configuration or cache.

Postgres version: 16 Spring-boot-starter-data-r2dbc version: 3.3.0