This stressed me out quiet badly for a month or so so thought i would share incase it helps. I was morbidly obese and am nearly out of the obese range after losing 55kg over last year. My resting was around 55 at the start of the year and it now dropped to about 39-43 after the first 20kg lost and has stayed there. Apple watch does not appreciate it being below 40 for 10mins+ and sounds the alarm about 3-4 times a day.
All other stats in the right range (blood pressure, blood sugar, cholesterol). Had to get anesthetic for a minor surgery a couple months after being like this and they did not seem concerned. Heart rate was higher though due to the infection. Asked the surgeon afterwards if its a thing to worry about and said as long as there are no symptoms such as lack of energy, dizziness then may be just a side effect of the weight loss. I do know if i push myself on exercise then my resting heart rate climbs back up to the 50s for the rest of the day. Obviously something that flies in the face of your medical advice though.
So my current attitude is if no other symptoms and my fitness metrics carry on getting better with no funny business when i exercise (heart rate wont go up when i am trying to exercise or does not fall quickly when i finish) then to just see how i go and go for a check up in a few months once i have lost a bit more weight. My Vo2 max has climbed steadily as well from shocking for my age to just below average so feels like there cant be to much wrong with me.
I suppose the question for you though is if this is something new and can you think of any lifestyle changes since it fell that may affect it? Also if truly worried buy a cheap heart fitness device (old apple watch, fitbit etc) to let you track it while you wait for your appointment. I know they are only like 85% accurate of something but you can at least see the trends.