Heart rate accuracy for 5k run

I haven't run for a couple of months and then I got my new Garmin Forerunner 245 music, the 24x7 HR seems quite accurate and comparable with my old Vivoactive 3. 

Then I did 5k session twice on two separate days, wearing my old VA3 at the same time, and the HR reading seems consistent with my VA3 too.

BUT, both of them showing me working out at 175-185 bpm for 25min of my 26min run... Usually (in the past), it starts in zone 2 and keeps in 3 for most of the time, then a bit in zone 4. Now it is almost zone 5+ all the way. I did not feel stressed much, and I do not think my heart is strong enough to maintain 180ish for 25min? (I am 37, male, 82kg)

Anyone else has similar experience? I have followed how to wear the watch for best HR reading guide. My VA3 used to give me more realistic readings but gives same results as the new FR245.