Retrofit or Jackson ObjectMapper maps "aId" property to lowercase "aid"

Try this @get:JsonProperty("aId")

See Michael Ziober' posted link for answer Usage of Jackson @JsonProperty annotation for kotlin data classes

Described issue is a result of Jackson's default bahaviour to not scan private fields. This behaviour can be change with @JsonAutoDetect

@JsonAutoDetect(fieldVisibility = Visibility.ANY)
data class MyClass(
   @JsonProperty
   val aId: String? = null, 
   @JsonProperty
   val abId: String? = null 
)