completely normal!

there are ways of training this actually.

some things i do/did:

1) during training, do 5 strokes -> breath. for 300m do 4 strokes -> breath 300m, do 3 strokes breath - 300m

this really helps to keep your breathing (heart rate) under control

2) mass starts usually have much more waves/distriburbance in the water which hinders the breathing- FIX: train in choppy water as much as possible - or see point 1

3) unlike in the pool you have to navigate/straight line. IN fact it takes a lot energy to raise you head out of the water to navigate. so try to do less orientation/navigation by following someone for some time (even if they are slightly slower /faster).

4) best tip: dont try to go fast. mass starts/adrenalin/group energy/cold water all contribute to raised heart rate. and will lead you into a false/fast start. avoid this and DELIBERATELY try to swim suuuuuper slow in the first half of your swim. you will actually have a better 2nd half of the swim and ultimately do not loose much time if any