Interestingly enough, this never happened riding the bike outdoors, or when wearing tight shirt - even on turbo-trainer.
Once I observe the malfunction, I always try to move HR strap (Soft Strap Premium Heart Rate Monitor), lift it, rub/press it against the chest, etc. After this kind of intervention, after few seconds, correct readings - understand: expected, more reasonable values, usually get back, but then disappear again. Using the graph from two days ago - after 51 minutes the HR steadily decreased to 69bpm and remained as such until end of exercise. This despite I was unwaveringly increasing cadence in last 5 minutes of the workout well above 100.
Does this have to do anything with me sweating and the shirt being completely soaked over the strap (Soft Strap Premium Hear Rate Monitor) and sensor (Plastic Heart Rate Module)? The watch is always same. The strap is always same. The module is always same. Bike is same. The only difference may be the clothing and the fact that when the readings are faulty - the bike is stationary... Is there a problem with wireless transmission from HR Module? Or is there problem with contact with profusely sweating pectorals, or is there a problem with soaked shirt above HR Module? Checking GARMIN Support online did not provide any relevant answer. May be the closest is "Try a cotton shirt or wetting your shirt: synthetic fabrics can rub or flap against HRM creating static electricity which can interfere with HR signals." Yes, I use synthetic shirts - but I use these riding indoors and outdoors, and in both cases these are pretty "wet" already by the time the problem occurs.
The issue is that these unreliable measurements impact values of "Average HR", "Calories" spent and "Training Effect" of particular workout, and obviously subsequent statistics... This does not make data obsessed athlete very happy...