One thing that differs from person to person is heat acclimatisation, only you as the stakeholder can answer that i guess. It's not going to be factored in by your training program or your smartwatch.
The perceived effort (ie RPE) for gauging zone 2 can also be somewhat gauged by trying to speak/sing.
See if you can complete a 10-15 word sentence if your HR is elevated on a warm day.
Alternatively, what you can also do now is also to try easy running in a gym, best if there is a fan. I used to do that when the company gym was available to me, in fact 1 standing fan and 1 floor fan. lol

Temperature pretty cold there, 20 deg C RH low 50s% according to the wall thermo/hygrometer.
You can then also solidify what RPE at Zone 2 feels like, so that you can then run on warm days (evenings like 6.30pm?) and see if you can form any relationship with regards to this cardiac drift.
Or just do it my wah lah, use chilled water and ice slushies, that 100 plus towel that you get from local races,

Coz ultimately whichever way you and I and anyone here cuts it, the high heat index warm weather is here for most parts of the year. LLST, we live here where it is warm and humid, ie high heat index.
For me i would never be able to run below 4.00 pace for more than that first few km if not for active cooling.....eg my MHR is around 180 (i measured 182 before, once), then i am already hitting like mid 170s past a certain km mark at say "just" 4.00-4.15, then push simi?