Heat acclimation going down training in high temps

It’s summer in Sydney and I am training in temps well above 22 degrees 6 days a week but my heat acclimation is going down with each run. I have just done a 40 min easy pace run in 28 degrees. I am now at 1%!  

The right (or at least close enough) temp is recorded in my app and Connect. Why is it going down?

  Photos from today’s stats. It was a base run with ave HR 139. I’m wearing an HR chest strap so it’s not like my HR is high to make it look like struggling against heat.