Martin neck resets: A warranty repair....what went wrong?

A warranty repair on a neck reset: what went wrong? I finished this in Feb '23 and it was fine for 3 years and then developed a heel gap and resulting high action. I asked customer to send it back and we'll find out what went wrong. Tour of steaming setup and neck jig along the way. Here's the customer's playlist of the original repair and in rewatching "My Instincts Were Right", I see that the guitar had a heel gap then and all I did was remove and refit the neck. Interesting!!!    • My HD-28   (00:00) What's the problem? (02:00) 2022-23 were ROUGH years (06:00) Removing the neck (11:55) My steam setup (21:45) The neck press (23:35) Steaming (24:30) What went wrong??? (29:15) Something funny (30:05) The neck jig (34:15) The neck relief is off (37:15) Check the action again (39:35) How much shim? (42:30) Fitting the neck (44:10) Stringing up: no glue (51:30) The next day (53:00) I think the problem was...