I am a long suffering Suunto strap user.
From new they work perfectly for 4 or 5 months. Then they start to record 30/40/50 bpm over the actual. Usually downhill. It is the strap not the sensor or watch. Messes your records up somewhat. Frequent washing with soap helps but it will never be properly reliable again. If under warranty send the strap back.
When it starts to go bonkers on a downhill I immediately unclick one of the 2 sensor connectors to the strap. No hr recorded for a bit but it is better than 230 bpm recorded. When the work gets hard again, click it back in and it always comes back to “normal”. Not perfect but workable.