Double count HRMRun when still connected and Wrist HR post exercise for a short period

I noticed the same thing with the HRM-Tri. When you remove the HRM, it takes some time for it to shut down. If it is wet, it takes longer for the HRM to register it isn't getting input signals, so it does not shut down. In the interim the watch may not have a solid OHR reading yet, and may not switch over to the OHR, thus still displaying garbage data from a HRM that has lost its input. Either way, it doesn't display valid measurements.

You could try prevent the transmissions from the HRM reaching the watch (make sure there is so much distance between HRM and watch that the watch can't receive it, or put the HRM in a metal box). Or make the HRM shut off quicker (rinse, wipe dry really well). Perhaps switching the watch from the mount to your wrist earlier, works also (it has more time to start obtaining OHR readings).