NGSI-LD option="keyValues" does not always work

[djs0109]

Describe the bug
It is reported that for get_entity_list if option="keyValues" is used, no entities can be returned

To Reproduce

from filip.clients.ngsi_ld.cb import ContextBrokerLDClient
from filip.models import FiwareLDHeader
from filip.models.ngsi_ld.context import ContextLDEntity, ActionTypeLD

attr = {"testtemperature": {"type": "Property", "value": 20.0}}
entity_list = [ContextLDEntity(
    id=f"urn:ngsi-ld:my:id{i}", type="MyType", **attr) for i in range(10)]


fiware_header = FiwareLDHeader(ngsild_tenant="test")
client = ContextBrokerLDClient(
    fiware_header=fiware_header, url="http://134.130.49.13:1027"
)

client.entity_batch_operation(entities=entity_list, action_type=ActionTypeLD.UPSERT)

entities_normal = client.get_entity_list()
entities_key_values = client.get_entity_list(options="keyValues")
entities_sys_attrs = client.get_entity_list(options="sysAttrs")

print("\nentities_normal", entities_normal)
print("\nentities_key_values", entities_key_values)
print("\nentities_sys_attrs", entities_sys_attrs)

client.entity_batch_operation(entities=entity_list, action_type=ActionTypeLD.DELETE)

entities_key_values is reported to be None sometimes. However, I cannot reproduce in my environment.