Question regarding JSON Template timestamp rendering

Hi friends,

I have a JSON Template layout in a project I maintain, and I have noticed a behavior in how timestamps are rendered that I did not expect. I went through the JSON Template resolver source for a few minutes last week and did not find an explanation, nor did I find any open issues or related messages on this list. I would greatly appreciate your help!

I am using Log4J 2.14.1 in a Java 11 project, and the following log lines were generated via a Docker-based test environment. I have a JSON Template configuration like:

```
{
  "timestamp": {
    "$resolver": "timestamp",
    "epoch": {
      "unit": "nanos"
    }
  },
  "timestamp.iso8601": {
    "$resolver": "timestamp",
    "pattern": {
      "format": "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'",
      "timeZone": "UTC",
      "locale": "en_US"
    }
  },
  ...
}
```

My log4j2.xml is fairly unremarkable, I think, like:

```
<Configuration status="WARN" packages="path.to.an.unrelated.custom.lookup">
    <Appenders>
        <Console name="LogToConsole" target="SYSTEM_OUT">
            <JsonTemplateLayout eventTemplateUri="classpath:myJsonTemplateLayout.json"/>
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="package.specific.logger" level="info" additivity="false">
            <AppenderRef ref="LogToConsole"/>
        </Logger>
        ...
        <Root level="debug">
            <AppenderRef ref="LogToConsole"/>
        </Root>
    </Loggers>
</Configuration>
```

Normally, the JSON template above produces log lines like:

```
{"timestamp.epoch":1634943559640000000,"timestamp.iso8601":"2021-10-22T22:59:19.640Z",...}
```

And as expected, the epoch timestamp matches the ISO timestamp shown.

Sometimes however, and frequently enough to prompt this message, I see the timestamps diverge by several milliseconds:

```
{"timestamp.epoch":1634943499616000000,"timestamp.iso8601":"2021-10-22T22:58:19.644Z",...}
```

The epoch shown here corresponds to 22:58:19.616, a drift of 28ms.

Additionally, I have seen that sometimes logs will share the same epoch timestamp, while their ISO timestamp increments:

```
{"timestamp.epoch":1634943424218000000,"timestamp.iso8601":"2021-10-22T22:57:04.533Z",...}
{"timestamp.epoch":1634943424218000000,"timestamp.iso8601":"2021-10-22T22:57:05.622Z",...}
```

The epoch shown here corresponds to 22:57:04.218, a drift of more than 300ms. In testing elsewhere, I also witnessed the same epoch timestamp used across logs up to 20 seconds apart.

Is this divergence and timestamp-repeating a known behavior? What might be occurring here?

Please let me know if I can provide more information to help clarify the situation, and thank you for any help on the matter.

Andrew