Why does jackson convert ZonedDateTime to decimal?

Here is the answer that gives a precise solution to your question: Spring Data JPA - ZonedDateTime format for JSON serialization. In short, you need to annotate your ZonedDateTime as follows.

ZonedDateTime time = ZonedDateTime.now();

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd'T'HH:mm:ss.SSSZ")
public ZonedDateTime getTime() {
    return time;
}

You also might need to add the dependency for JSR-310

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-jsr310</artifactId>
    <version>2.6.0</version>
</dependency>

If you use Spring Boot, you can solve it in a more general way with the following property:

spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS=false

This way you don't need to specify a JsonFormat for each date field you have.

If you also want to customize the date format globally, use the property:

spring.jackson.date-format

Source: https://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html

I prevented the conversion by injecting MappingJackson2HttpMessageConverter in the Integration Test

MockMvcBuilders.standaloneSetup(myController).setMessageConverters(mappingJackson2HttpMessageConverter).build();