Rival Heart Rate - Erratic periods

Recently I’ve noticed that my setup is reporting erratic heart rate for short periods of time during a ride.
I’ve got an Elemnt and a Roam. I’m road riding without aero bars (although this has happened during an off road ride).
There’s a period of no heart rate reported followed by a period of lower-than-should-be heart rate.

I can see when this happens because the LEDs on my roam are set to heart rate and it goes to one blue LED. I’ve not correlated this to any particular position on the bike. It’s always a decent percentage into the ride.

I’ve been back through my workouts and can only see it happening since 29th June 2024.

Traces added for illustration.

Has anyone else had this issue? Does anyone know what’s happening and if there is anything I can do differently to fix this issue?