It is a garmin HRM linked to my Edge 530 and watch. Not a holter monitor but I’d guess fairly reliable. And again, the data are consistent. MTB ride => very high HR. This monitor also picked up the bradycardia which sent me to the ED. The day after that ridiculous race where 9L of electrolyte drink/CHO were not enough to keep me hydrated my HR was dipping to the low 40s sitting down (after coffee/food etc). Prolonged QT on EKG. Resolved immediately after the (lab diagnosed) low K and low Mg were replaced.
My wife always had problems when she wore a HRM under her bra. She went with a wrist HRM instead with much better results. But, she had that problem regardless of terrain, so not sure it applies here.
I’m a nurse. I know K is nothing to toy with. The ER was not normal, even for me 
That was secondary to a very bad day in very bad conditions. That was dehydration.
And I do have a cardiology follow up… in September 
Next time it is happening take your pulse manually.
The theory about a bad hrm reading is based on it being mtb specific, perhaps related to the tougher terrain than you see on the road or indoors. In addition to manually taking your hr you could also try wearing two hrms or positioning it differently
Thanks to covid, I have a little pulse oximeter I’ve used a couple times to confirm my Wahoo Tickr. Easy enough to toss in your pocket. My pulse ox matches the Tickr really well during inside riding (which always has surprised me due to finger vs. chest strap).
Caveat this heavily with ‘speak to your doctor’, but have you had a blood test done recently? The K and Mg issues are a bit of a flag for me- dehydration caused by sweating is mostly sodium-related, and generally speaking you shouldn’t see other electrolytes depleted to the kind of levels where you’d need an IV over the course of a day unless they were significantly low to begin with. While I can’t say with any certainty if that’s the case, chronically low levels will absolutely cause an elevated HR, as well as a whole host of more severe problems in extreme cases. (but relatively easy to diagnose/fix)
Iron could be worth looking into as well, especially if you’re female and a higher-volume rider. I ran into a deficiency a while back and had high HRs for a while before I picked up on any other symptoms. (Things like fatigue are gradual and kind of vague, so easy to miss early on IME)