I can’t figure out this website. Maybe it’s because I’m looking at the mobile version?
Can you look up temp on DL838 2/26?

I don't think anyone keeps details like that. You'll only find out the aircraft registration. There are other resources that might detail any mechanical faults that might register. But DL838 on 2/26 was done by N6709 (SLC to SEA). One interesting note was that there is an incomplete entry right after before leaving for MCO which might indicate the airplane might have been taken out of service for an issue and repaired overnight. What was the return flight and when? (Keep in mind, the free version has a seven day history).

https://www.flightradar24.com/data/aircraft/n6709

This is mostly cultural.

US based flyers (and hence US carriers) are accustomed to full air conditioning all summer long -- much of the country (exceptions are Alaska, the PNW, and northern New England) is mechanically cooled in the summer months. This also means the air is dryer -- air conditioned cooling also means dehumidification.

In Europe, air conditioning is less common. When I lived in Germany, the FSU, and other parts of northern Europe, I noticed that people kept rooms much warmer than I was accustomed to (21C is fairly standard in the US, Europe often sets the temp somewhere around 23 or 24). People also wear lighter, more breathable clothing nowadays, making them colder and requiring more room heat.

The few places where workers still wear wool suits every day (banks, mostly) will often be kept much cooler than other office environments.

I can't speak for European, but Asian flights are generally kept much warmer... those that sit near the exits tend to be shivering a lot even under two or more blankets due to the air leakage. In the before times, I kept a USB powered fan in my pack in case they set the cabin temperature higher than I would like.

But in this case, I don't think it applies. It's a US domestic flight. Likely the air conditioning unit was malfunctioning. But until the OP comes back with the other flight (and see if it is still within a recallable history) we won't know.

It's not air leakage. If any air leaks, it is cabin air leaking out. The cold outside air can't leak into the pressurized cabin anymore than water can leak up a waterfall.

It is cold around exits and doors because the doors cause breaks in the insulation and the materials around the area lose heat more quickly via conduction creating a colder interior wall.

That was our return flight. Outbound is more than a week ago.
So the sauna plane went in for service right after our flight. I guess there’s no way to know if the temp issue was the reason. But I’m telling you it was a hot flight, many pax had stripped down to T-shirts, numerous babies screaming.

I woke up on SIngapore airlines once in a sweat as it was so hot. In my sleepy state I called out..."Why is it so hot in here"!

Anything happen in response to that?

David