High HR concern on runs

I got a Garmin 255 several months ago and have been able to track my HR while running for the first time. I ran a half marathon (my 4th) in April and am now starting marathon training (my 2nd). I’m seeing pretty high HR even on easy runs and am starting to wonder if this a medical concern?

Maybe this is due to summer heat and humidity? Or dehydration? Or my watch just measuring wrong? Would appreciate any thoughts!

Some context:

-I’m 26 (max HR ~194bpm per 220 rule) and have been running since I was 12; my RHR is ~60bpm

-I’ve noticed more and more on runs that my HR seems to start ~130-140 and creep steadily up towards 170-180 even on recovery days where I’ve pushed myself to go deliberately VERY slow, much slower than I’ve ever run before

-My watch says my average HR during my half marathon in April was 180 and my HR was in Zone 5 for 90% of the race - is this even possible?