`ZonedDateTime` not serialised as UTC by default altough ObjectMapper Javadocs claiming UTC is the default

As stated in the Jackson ObjectMapper the default should be UTC for writing Json:

/**
* Method for overriding default TimeZone to use for formatting.
* Default value used is UTC (NOT default TimeZone of JVM).
*/

public ObjectMapper setTimeZone(TimeZone tz) {
_deserializationConfig = _deserializationConfig.with(tz);
_serializationConfig = _serializationConfig.with(tz);
return this;
}

https://github.com/FasterXML/jackson-databind/blob/b8910125bba81862ea278821ccfd198b1f69fd5d/src/main/java/com/fasterxml/jackson/databind/ObjectMapper.java#L2529-L2537

/**
* Base settings contain defaults used for all {@link ObjectMapper}
* instances.
*/
protected final static BaseSettings DEFAULT_BASE = new BaseSettings(
null, // cannot share global ClassIntrospector any more (2.5+)
DEFAULT_ANNOTATION_INTROSPECTOR,
null, TypeFactory.defaultInstance(),
null, StdDateFormat.instance, null,
Locale.getDefault(),
null, // to indicate "use Jackson default TimeZone" (UTC since Jackson 2.7)
Base64Variants.getDefaultVariant(),
// Only for 2.x; 3.x will use more restrictive default
LaissezFaireSubTypeValidator.instance,
// Since 2.12:
new DefaultAccessorNamingStrategy.Provider(),
// Since 2.16: [databind#2502] Add a way to configure Caches Jackson uses
DefaultCacheProvider.defaultInstance()
);
https://github.com/FasterXML/jackson-databind/blob/b8910125bba81862ea278821ccfd198b1f69fd5d/src/main/java/com/fasterxml/jackson/databind/ObjectMapper.java#L408-L422

With the java 8 time module when the timezone is set to null no time zone information is used on serialisation as the provider.getConfig().hasExplicitTimeZone() does result in false if the default of null is set. So ZonedDateTime is not written with utc as default.

https://github.com/FasterXML/jackson-modules-java8/blob/38f979fbe16a03344f08d6ca899204b7c3bc1afb/datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/ser/InstantSerializerBase.java#L147-L163

Is this expected behavior to not overwrite the TimeZone defined in the ZonedDateTime? Than this should be properly Documented.

Minimal example:

ObjectMapper testmapper = new ObjectMapper();
testmapper.registerModule(new JavaTimeModule());
testmapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
ZonedDateTime testtime = ZonedDateTime.now(ZoneId.of("Europe/Berlin"));
String teststring = testmapper.writeValueAsString(testtime);
// 2024-02-20T11:37:42.009045+01:00

testmapper = new ObjectMapper();
testmapper.registerModule(new JavaTimeModule());
testmapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
testmapper.setTimeZone(TimeZone.getTimeZone("UTC"));
testtime = ZonedDateTime.now(ZoneId.of("Europe/Berlin"));
teststring = testmapper.writeValueAsString(testtime);
// 2024-02-20T10:38:13.0324177Z

Comece pelos Javadocs de ObjectMapper.DEFAULT_BASE e setTimeZone e, em seguida, inspecione datetime/src/main/java/com/fasterxml/jackson/datatype/jsr310/ser/InstantSerializerBase.java, especialmente o caminho provider.getConfig().hasExplicitTimeZone(). Reproduza os dois casos de ZonedDateTime da issue com JavaTimeModule e WRITE_DATES_AS_TIMESTAMPS desabilitado; considera-se concluído quando o padrão UTC pretendido estiver esclarecido na documentação ou coberto por um teste.

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