I have had the same problem with the HRM Tri. It stopped working after the second battery replacement.
It probably lasted 9 months, pretty disappointed as I love the Garmin watches and head units but the next HRM strap I buy will probably the the wahoo one.
That being said, I have found that the older Garmin HRM that you can clip the transmitter to works well and has not stopped working yet… (3 years)