I'm using the HRM PRO while playing floorball, with my Instinct 2S Solar on the sidelines. I start a custom activity without GPS on the watch and then leave the watch in my bag but after saving the activity my HR graph looks like in the image below, with HR readings missing around half the time. As I understand it, the HRM PRO and my watch should both support the "store and forward" feature (support.garmin.com/.../, and the watch does download HR data when saving the activity, so I don't know what I'm doing wrong. I've tried manually downloading the data but it does that instantaneously and does not create a new activity file. I can only see the original activity file when plugging in my watch to my PC.
Any ideas what I could be doing wrong here? And is there anything I could try to fix this?