startTs mismatch when running multiple upserts within one transaction

[Grandys]

Dgraph4j version: v2.1.0
Dgraph version: v1.2.0

I got following error when running multiple upserts within single transaction:

java.lang.RuntimeException: java.util.concurrent.CompletionException: java.lang.RuntimeException: The doRequest encountered an execution exception:
....
Caused by: java.util.concurrent.ExecutionException: io.dgraph.DgraphException: startTs mismatch
	at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1908)
	at io.dgraph.DgraphAsyncClient.lambda$runWithRetries$2(DgraphAsyncClient.java:180)
	... 6 more
Caused by: io.dgraph.DgraphException: startTs mismatch
	at io.dgraph.AsyncTransaction.mergeContext(AsyncTransaction.java:252)
	at io.dgraph.AsyncTransaction.lambda$null$0(AsyncTransaction.java:165)
	at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616)
	...

Sample code:

DgraphProto.Operation op =
        DgraphProto.Operation.newBuilder()
            .setSchema("email: string @index(exact) @upsert .")
            .build();
dgraphClient.alter(op);

JsonArray jsonData = new JsonArray();
JsonObject person = new JsonObject();
person.addProperty("uid", "uid(v)");
person.addProperty("name", "wrong");
jsonData.add(person);

JsonObject person2 = new JsonObject();
person2.addProperty("email", "[email protected]");
person2.addProperty("uid", "uid(v)");
jsonData.add(person2);

String query =
    "{\n"
        + "    me(func: eq(email, \"[email protected]\")) {\n"
        + "        v as uid\n"
        + "    }\n"
        + "}\n";
Mutation mu =
Mutation.newBuilder().setSetJson(ByteString.copyFromUtf8(jsonData.toString())).build();
Request request = Request.newBuilder().addMutations(mu).setQuery(query).build();

Transaction transaction = dgraphClient.newTransaction();

// This upsert works
transaction.doRequest(request);

try {
  transaction.doRequest(request);
} catch (RuntimeException e) {
  transaction.discard();
  fail(e.getMessage());
}

Problem does not occur for multiple Queries and Mutations. When running queryWithVars or mutate, new Request object is build and property startTs is inherited from context. When raw Request is passed into doRequest it does not happen, so this may cause the problem?