Hi Canopus!
Thanks for responding and your suggestion.
I cleaned out / deleted all the current events for the track, then dragged in a new 1 bar beat from Ez Drummer and let it play in a loop. The first play-through also does not play the first note, but once in the loop, it does. For the 1 measure clip there were 13 events, and no other events before the beginning of the track.
However, I noticed that whilst auditioning a clip in Ez Drummer, the first note is also silent. Hmm... To me that indicates the problem being somewhere with the audio interface (Behringer 404HD), it's associated ASIO driver, or hidden in the depths of Windows. Something else that may shed some light on the problem, I noticed that although there is no sound from the first note, the track's volume meter does respond.
Regarding Windows, I installed Process Lasso to assign a higher priority to Windows' audiodg.exe which handles all its audio processes, and assigned it a high priority. (This was a suggestion from a a Youtuber worth trying.) Sadly there was still no difference. I also reinstalled Behringer's ASIO driver (again), to no avail.
The mystery deepens, and I can feel my Grammy slipping away...