Nightly HRV data periodically missing

Question

My Index Sleep Monitor periodically doesn't sync nightly HRV data to Garmin Connect. The HRV graph is also empty, though stress data is there. Is this a bug or a hardware issue?

Garmin - KS · Answer

Occasional missing HRV data does not necessarily indicate a problem with the device. Please ensure that the Index Sleep Monitor module pocket is laying flat against your skin, and that it isn't too tight. This FAQ offers some best practices that may be helpful: Best Practices When Wearing the Index Sleep Monitor 
 Of course, we also recommend ensuring that the Garmin Connect app remains up-to-date, and to allow the device to fully sync each morning. If this issue becomes frequent or persists, please let us know!