Not unusual. Maybe a little on the low end of the distribution, there can be great variability among athletes when it comes to heart rate response.
One thing to consider…as training load goes up blood volume will increase. At increased blood volume you can maintain similar work rates at lower heart rate. You can review the Coyle/Coggan work on performance vs blood volume to get an idea of how this interaction behaves among trained athletes (probably the founding papers for blood doping 
). So maybe after your basecamp you have achieved some new level of fitness & you are just able to ride the same Z2 power at a lower heart rate.
Of course, depressed HR can also be a sign of overreach/overtraining…so there is that to consider. With your age, workload, and prudent intensity schedule I find it unlikely that you are overtrained.
Garmin VO2max estimates can be wildly inaccurate on an absolute basis. I think it’s good for letting you know if VO2max is INCREASING or DECREASING…but not for letting you know what it actually is. So please don’t waste any mental energy regarding this. You are doing just fine. Next time you finish ahead of somebody in a race be sure to let them know your Garmin says your VO2max is less than 50. They will love to hear it.