I have the same thing going on.
Born and lived at sea level for 40+ years, then 10+ years at moderate altitude (Boulder)...
Now in the middle of a two-week training break at sea level (Eugene, not just sea level but LOTS of moisture in the air)...
I expected to be able to train at a lower heart rate doing the same amount of work: yes. HR averaging 124 instead of 130-134. I did NOT expect to see framatic fall in resting HR almost immediately.
"Resting" HR (actually monitored while sitting in the car on the way to trailheads/tracks) is usually 55-59-ish; in Eugene it has been 45-48 or lower even - and this was from second day.
I've visited sea level before, but never while continuing a consistent aerobic training phase.
My reluctant conclusion is that I have never properly adapted to altitude (despite covering all the hydration, ferritin etc bases - I had iron infusions first two years), and I've since been told that when it comes to altitude it is "well-known" there are responders and non-responders.
Actually I think it's not just a question of being a non- or partial-responder, but that for some of us, altitude is actually "harmful" in some way.