Investigation revealed the same problem in both airplanes. On the firewall is a mixing box that mixes the heated and ambient air before it enters the cabin. There are a couple of electrically actuated valves that control the flow of heat into the system. In some cases, these were improperly rigged and the valve allowing heat into the system didn't close completely which means warmer than normal air enters the cabin.
This issue is less likely to be noticed in the winter when you want the higher heat, but it is more likely to be seen as inadequate cooling as with heated air consistently leaking into the vent air (or conditioning air), the perceived cooling performance would be anemic.
We recently released Service Bulletin SB2X-21-05 which inspects for this valve not properly closing, and provides rigging instructions to ensure it is properly closed.
