I had a something similar happening and coincidentally, I was on the beach too! Everytime I tried to change the height, the drone would want to land. Then an even worse thing happened - after taking off, it started to keep going up! I could not do anything and I thought it would keep rising till it stopped and then ran out of battery and fell back down. It took a lot of RTH commands to slowly bring it down.
It all turned out to be a stick problem and I believe the ones in the MP controller are very sensitive because they use magnets. So when I sent RTH, the stick was sending a signal as well and preventing it from coming down as commanded.
I was very bothered by the stick matter and after many months of trying different things, I now believe that the stick protector (the long aftermarket accessory that is placed over the sticks to prevent them from being pushed out of place when stowed) is not recommended. I keep my RC in a case and everytime I had the protector in place, I would get stick error when I started the RC and it would take a long time to go away. For the past 5 sessions where I have not used the protector, I can take the RC out, switch it on and be flying with a couple of minutes. No more stick error!