Garmin short heart rate transmitter strap lifespan

Start using the new strap a year ago. This was to last 3.5 years. Recently, Jan 2020, after about an hour of riding the transmitter showed an abnormally low reading. I checked the battery, which was not exhausted, but I installed a new one just to be sure. On the next ride exactly the same thing happened. Then on my next ride I used a new transmitter purchased in (fall 2019), and it worked normally. My rides are of normal duration and frequency so the transmitter show have lasted at least for 3.5 years.

Any info or recommendations appreciated.