HR during swim keeps fluctuating v20.xx

The HR during swimming keeps fluctuating (even with the latest updates - v20.xx). The watch is perfectly fitted on the hand and there's no issue with the skin or gap between the sensor and the skin. It stays shallow for the most part and then suddenly, it will correct itself. While, (1) I know that after 4 laps, my HR cannot be 80s but sure 130 to 140, the watch shows me HR in 80s and then after a while, it suddenly corrects itself but goes to 140s. On the other hand, I started wearing my older FITBIT to compare the HR in real-time. Fitbit shows me in the range of 130 while Garmin keeps showing me HR in the range of 80. Is anyone else also facing this issue?