I too never had much drift (age 64, always used chest strap). I stopped testing after averaging 152 and then 156 in successive stages, during my most recent test in October. These were clearly way higher than I could endure regularly, so I realized that I am one of those for whom the HR drift test, as conventionally performed, does not provide a meaningful reading of AeT.
Do you have a special diet? I ask because I eat very low carb. I do not do it for athletic performance reasons, so I am not proselytizing. I just believe that a lot of the measurement norms that work for most trainees, seem not to work for the few people on this site who drastically restrict carbs.