Either it is electrical interference by the Navy, a software glitch, or your HRM strap is going south. I have Garmin as well and my strap started sending wonky readings to the transceiver unit. I would be riding along and my heart rate would show 115 when I knew it should be above 130. If it starts to happen in other places, replace the strap. If that is the sole place, its EMI.
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Road 🚴🏾♂️ & Mountain 🚵🏾♂️