Zwifting indoors ≠ riding a bicycle outside.

I pay as close attention to HRR as anyone, and can say that dehydration is factor #1 in recovery rate... exactly as I said back in 2018. If your heart rate stays elevated after effort, take in more water.

No matter how much you drink, if you put in particularly hard efforts, it's still going to happen every once in awhile. I don't even need to pay attention to the numbers until I'm into my 3rd or 4th hour, at least at this time of year. I can't use up
the water I've got in an hour or two. I could go absolutely full gas for an hour (like an FTP effort) without a drop of water and be absolutely fine-- I think most people could.

If someone gets elevated heart rate from riding for an hour at any intensity... to the cardiologist with thee.