You are using bluetooth for HR broadcast (no ant+). Bluetooth is a one-to-one protocol. Your phone may have grabbed the bluetooth connection. Disconnect your watch from any apps that could be using the bluetooth connection.
Pixel (google) also went security nuts for HR broadcast. Per DC rainmaker " ...basically anything that accepts a heart rate strap will probably, maybe, hopefully, accept the Pixel Watch data stream. I say that with all those caveats because Google has made life far more difficult than it needs to be, and required access authorization each time you want to use the HR sensor with a 3rd party device" and that device needs to understand the Bluetooth GATT security protocols.
A HR strap/belt is a pain to use, but is more accurate than wrist based HR while rowing. Rowing seems extra hard for wrist based devices (vs cycling, running, etc). HR belts range from really cheap to $70 for excellent products that are precise enough for HRV analysis. I use a belt instead of relying on a very accurate garmin 265 watch for HR data - it just works better.
https://www.reddit.com/r/PixelWatch/com ... broadcast/
Here's the complete list for anyone wondering what the Pixel Watch 3 can broadcast heartrate to:
Connectivity has been tested and confirmed for the latest models of:
Peloton Bikes, Treads and Rows
Concept 2 (with latest firmware version only)
Echelon
Hydrow
iFit (NordicTrack)
Matrix
Schwinn
Spinning
Strava
Tonal
Wahoo
Zwift
Known manufacturers that are not compatible with Pixel Watch 3:
Garmin
Life Fitness
TechnoGym