"data" set via TransactionTracer.SetData is not written to "Data" of the Trace-Context, and does not get copied over to SentryTransaction when created from the tracer.
But "data" set via TransactionTracer.Contexts.Trace.SetData is written to "Data" of the Trace-Context, and gets copied over to SentryTransaction when created from the tracer.

However, SentryTransaction.SetData and SentryTransaction.Contexts.Trace.SetData are equivalent setting "Data" of the Trace-Context.
With Spans, both setting "data" via SpanTracer.SetData and SentrySpan.SetData are eventually serialized to spans.data ("Data" does get copied over to SentrySpan when created from a SpanTracer);

Steps to Reproduce

SentrySdk.Init(options =>
{
    options.Dsn = "..";
    options.TracesSampleRate = 1.0;

    options.SetBeforeSendTransaction(transaction =>
    {
        transaction.SetData("setbeforesendtransaction", "transaction-setdata");
        transaction.Contexts.Trace.SetData("setbeforesendtransaction", "transaction-contexts-trace-setdata");
        return transaction;
    });
});

var transaction = SentrySdk.StartTransaction("my-transaction", "start-transaction");
SentrySdk.ConfigureScope(scope => scope.Transaction = transaction);
transaction.SetData("transaction-data-key", "transaction-data-value");
transaction.SetData("transaction-array", new[] {42, 43, 44});
// Do some work.
transaction.Finish();

Expected Result

{
  "contexts": {
    "trace": {
      "data": {
        "setbeforesendtransaction": "transaction-contexts-trace-setdata",
        "transaction-array": [42, 43, 44],
        "transaction-data-key": "transaction-data-value"
      }
    }
  }
}

Actual Result

{
  "contexts": {
    "trace": {
      "data": {
        "setbeforesendtransaction": "transaction-contexts-trace-setdata"
      }
    }
  }
}

Additional Notes

• Uncovered through Updates the Unity SDK docs with new SetData methods on spans & transa… sentry-docs#13457