According to the manual, Edge shall give you a heat acclimatisation when the temperature is above 22 degrees Celsius. I did a 3.5hrs activity today and the temperature was between 22 and 27 degrees. Phone was connected the whole ride too and it had weather data on screen.
But I didn't got any heat acclimatisation info nor update and I can't see it in Connect.
Is this a bug or am I doing something wrong here?
for the heat acclimation the Garmin gets the temp data from the nearest official weather station, mostly airports (and not from your Edge temp sensor), so if your Edge recorded 22-27C during the ride but the official weather station reported 18C, you dont get heat acc... (you can check it on the web version of Garmin Connect, here: click on the temp and it will show more details with station code: 06690, it is St. Gallen Airport, Switzerland, it shows 18.9C, i started at 9:18, it was already 25C when and where i started, which was not in St Gallen, i was on the south side of the big hill, St Gallen is on the north side, which is obviosly cooler on the morning, i almost died so strong was the sunlight even so early on the morngin, and i didnt get any heat acclimation because the GC think i rode all the 3.5h in this cool 18.9C. So it is a totally idiot feature, especially where the terrain is very different even in a small circle (mountain regions, Alps), or where there the official stations are very rare, and only just a few nearby and not on the same altitude as you start the ride (i have another example too, there is 300m difference between the station altitude and my starting postion, and obviously the station on 300m higher in the mountain gives you minimum 3-4C or cooler forecast which makes this heat acclimation feature meaningless...
Wowsers, even lower than your temp. 
I've only had my Edge 1040 for a month, my old 1000 having finally failed me. A lot of features and analytics are therefore new to me, including this.
I have done a couple of very warm rides, but the unit's thermometer would have been registering below 22 degrees at the start of each, because the unit is stored indoors where it's cooler. I'd seen nothing on the device at all, but having read this thread I wasn't surprised.
However, I now note that the Training Status metrics at Garmin Connect (web browser version) include Heat Acclimation data:
