Sending MIDI using MIDISendEventList() in C/C++

I've been working to migrate an older Core MIDI sending implementation to send MIDI 1.0 messages using Apple's newer UMP-aware MIDI Event List API methods.

I've figured out code that runs and should output MIDI clock messages, but when I send it with MIDISendEventList(...) I see nothing being output from my MIDI interface; there's also no error returned from that method to indicate what the problem is.

Here is the code I'm using:

const ByteCount clockMessageSize = 1;
const UInt32 clockMessage[clockMessageSize] = { (UInt32)0xF8 };    // MIDI clock tick 
const MIDITimeStamp timeStamp = mach_absolute_time();
MIDIEventList clockMessageEventList = {};
MIDIEventPacket* clockMessageEventListEndPacket = nullptr;

clockMessageEventListEndPacket = MIDIEventListInit(&clockMessageEventList, kMIDIProtocol_1_0);
clockMessageEventListEndPacket = MIDIEventListAdd(&clockMessageEventList, sizeof(MIDIEventList::packet), clockMessageEventListEndPacket, timeStamp, clockMessageSize, clockMessage);
                
for (NSUInteger endpointRefIndex = 0; endpointRefIndex < endPointRefsCount; ++endpointRefIndex) {
    MIDIObjectRef destinationEndpoint = endPointRefs[endpointRefIndex];
    OSStatus midiSendError = MIDISendEventList(outputPortRef, destinationEndpoint, &clockMessageEventList);
                    
    if (midiSendError != noErr) {
        printf("MIDISendEventList error: %i", (int)midiSendError);
    }
 }

Inspecting clockMessageEventList.packet after it has been configured but before it is sent shows:

(248, 0, 0, [... all zeros to index 63]) 

Does anyone know where I'm going wrong?