diff --git a/libs/ardour/midi_playlist.cc b/libs/ardour/midi_playlist.cc
index 84587a1d88..1fe6dc42e5 100644
--- a/libs/ardour/midi_playlist.cc
+++ b/libs/ardour/midi_playlist.cc
@@ -357,6 +357,11 @@ MidiPlaylist::render (MidiChannelFilter* filter)
 	} else {
 
 		DEBUG_TRACE (DEBUG::MidiPlaylistIO, string_compose ("\t%1 layered regions to read\n", regs.size()));
+#ifndef NDEBUG
+		for (auto & r : regs) {
+			DEBUG_TRACE (DEBUG::MidiPlaylistIO, string_compose ("\t\t%1 on layer %2\n", r->name(), r->layer()));
+		}
+#endif
 		bool top = false;
 		std::vector<samplepos_t> bounds;
 		EventsSortByTimeAndType<samplepos_t> cmp;
@@ -370,6 +375,7 @@ MidiPlaylist::render (MidiChannelFilter* filter)
 
 			if (--n == 0) {
 				/* render topmost region as-is */
+				DEBUG_TRACE (DEBUG::MidiPlaylistIO, string_compose ("render top region %1\n", mr->name()));
 				mr->render (evlist, 0, _note_mode, filter);
 			} else {
 				Evoral::EventList<samplepos_t> tmp;
@@ -392,6 +398,7 @@ MidiPlaylist::render (MidiChannelFilter* filter)
 						mtr.resolve_state (evlist, slist, ev->time());
 					} else if (region_is_audible_at (mr, t)) {
 						/* no opaque region above this event */
+						DEBUG_TRACE (DEBUG::MidiPlaylistIO, string_compose ("region %1 is audible for event %2\n", mr->name(), *ev));
 						uint8_t* evbuf = ev->buffer();
 						if (3 == ev->size() && (evbuf[0] & 0xf0) == MIDI_CMD_NOTE_OFF && !mtr.active (evbuf[1], evbuf[0] & 0x0f)) {
 							; /* skip note off */