Unit test hangs forever if `DoFn` resets event timers - google-cloud-dataflow

Question

I'm unit testing (with TestStream and PAssert) a DoFn that resets event timers. Test hangs forever if DoFn resets timers and this behavior seems specific to event domain timers.
Is this a bug in beam testing facilities or expected timer behavior?
Here is a toy example that I can reproduce this behavior with beam 2.3 SDK.
static class KeyElements extends DoFn<String, KV<String, String>> {
#ProcessElement
public void processElement(ProcessContext context) {
final String[] parts = context.element().split(":");
if (parts.length == 2) {
context.output(KV.of(parts[0], parts[1]));
}
}
}
static class TimerDoFn extends DoFn<KV<String, String>, KV<String, String>> {
#TimerId("expiry")
private final TimerSpec timerSpec = TimerSpecs.timer(TimeDomain.EVENT_TIME);
#ProcessElement
public void processElement(ProcessContext context, #TimerId("expiry") Timer timer) {
timer.set(context.timestamp().plus(Duration.standardHours(1)));
final KV<String, String> e = context.element();
context.output(KV.of(e.getKey(), e.getValue() + "_output"));
}
#OnTimer("expiry")
public void onExpiry(OnTimerContext context) {
// do nothing
}
}
#Rule
public TestPipeline p = TestPipeline.create();
#Test
public void testTimerDoFn() {
TestStream<String> stream = TestStream
.create(StringUtf8Coder.of())
.addElements(
TimestampedValue.of("a:0", new Instant(0)),
TimestampedValue.of("a:1", new Instant(1)),
TimestampedValue.of("a:2", new Instant(2)),
TimestampedValue.of("a:3", new Instant(3)))
.advanceWatermarkToInfinity();
PCollection<KV<String, String>> result = p
.apply(stream)
.apply(ParDo.of(new KeyElements()))
.apply(ParDo.of(new TimerDoFn()));
PAssert.that(result).containsInAnyOrder(
KV.of("a", "0_output"),
KV.of("a", "1_output"),
KV.of("a", "2_output"),
KV.of("a", "3_output"));
p.run();
}
The above test would halt if input elements are a:1, b:2, c:3, d:4.