Setting Jackson feature WRITE_DATES_AS_TIMESTAMPS not working in Spring Boot

I set spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS=false in the Spring Boot config but the Jackson serializer still produces [1942,4,2] instead of "1942-04-02" for a DateTime value.

Some debugging snapshots

• In org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration.Jackson2ObjectMapperBuilderCustomizerConfiguration.StandardJackson2ObjectMapperBuilderCustomizer#customize there's

  configureFeatures(builder, this.jacksonProperties.getSerialization());

  which shows that "WRITE_DATES_AS_TIMESTAMPS" -> "false"

• Then a bit later in org.springframework.http.converter.json.Jackson2ObjectMapperBuilder#configure there's this loop

  for (Object feature : this.features.keySet()) { configureFeature(objectMapper, feature, this.features.get(feature)); }

  and again this.features says "WRITE_DATES_AS_TIMESTAMPS" -> "false"

• Yet during serialzation of a DateTime com.fasterxml.jackson.datatype.jsr310.ser.JSR310FormattedSerializerBase#useTimestamp says false because provider.isEnabled(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS) returns false.

Attempts at fixing

• Out of despair I replaced spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS=false with spring.jackson.serialization.write-dates-as-timestamps=false because I found that mentioned in a lot of places (even though the Boot documentation doesn't hint at this). What about this? They seem to be synonyms - no effect.
• While writing this question SO suggested WRITE_DATES_AS_TIMESTAMPS not woking on Spring boot 1.3.5. The answer says to replace WebMvcConfigurationSupport with WebMvcConfigurerAdapter. While this does help indeed I fail to understand why so.