HRV Issue (discontinuous lines)

I think these gaps are when the sensor isn't getting a good reading. The manual talks about cleaning the sensor window and adjusting the band to be not too tight and not too loose. But it also mentions that your sleeping position can interfere, such as compressing your arm so that circulation is reduced to your extremity or having weight pressing on the watch so that it compresses the skin behind the sensor too much (same as if the band is too tight).

When I look back through my history, I see some discontinuities in my HRV plots but nothing as severe as in your picture. But, I have some nights where it showed no data at all once in a while.  I don't know if that means I had the watch fit wrong that night or if it just failed to sync the new data.