I run a raised flood/drain tray in the tent, with a heater underneath. I’m forced to run the heater all year long due to low ambient temps.
I also have a Growlink system which monitors my substrate temps. Despite the heaters directly underneath the plants, the substrate temp is always 6-8f degrees lower than ambient air. So I don’t think we need to worry about that.
BUT - my heaters do increase my substrate temps by a similar 6-8 degrees, and I generally find that beneficial for growth.