Home Assistant card shows wrong value

Hello,

I have a MQTT temperature sensors using the automatic mqtt discovery. On startup, the sensor is sending the following MQTT messages:

Topic:
homeassistant/sensor/test7_home_heizung_hk2/Soll_Temperatur/config
Message

{
    "name": "Test7.Home.Heizung.HK2.Soll_Temperatur",
    "object_id": "Test7.Home.Heizung.HK2.Soll_Temperatur",
    "device_class": "temperature",
    "state_topic": "Test7/Home/Heizung/HK2",
    "unit_of_measurement": "°C",
    "value_template": "{{ value_json.Soll_Temperatur}}"
}

The sensor is sending value updates with the following MQTT message:
Topic:
Test7/Home/Heizung/HK2
Message:

{
    "Time": "2022-09-14T21:57:01",
    "Soll_Temperatur": 5
}

Now I have added this sensor to my Dashboard in a history-graph and an entities card using the following configuration:

type: vertical-stack
cards:
  - type: history-graph
    entities:
      - entity: sensor.test7_home_heizung_hk2_soll_temperatur
    hours_to_show: 1
  - type: entities
    entities:
      - entity: sensor.test7_home_heizung_hk2_soll_temperatur

The interesting thing is, that the history graph shows the proper value (here 5°C) and the entity card just shows 0°C.
Screenshot_1

How can that be?
When clicking on the entity card, the value history is shown. There, 5°C is visible.

Any ideas how to fix that?

Kind regards,
Gurkenscheibe

Here is the screenshot from the history when clicking on the entity card:
Screenshot_2

I just played around and found, that the graph from the last image only shows points. These points are not connected with lines. So maybe the entity card shows zero, because the value is only valid vor a very short moment? How can I tell home assistant to kepp the value valid until the next value is received from the sensor?