Can you point to the documentation where we state it’s not supported, then we can fix it?
If you use JsonNode/ObjectNode (on the Java level), it will be stored properly (which is what REST seems to use). All current possible types are here: flowable-engine/modules/flowable-variable-service/src/main/java/org/flowable/variable/service/impl/types at main · flowable/flowable-engine · GitHub (including JsonType).