"Terminator Rules" Time Travel, using Skynet-branded Time Displacement Equipment (TDE), from all relevant sources, tells us that:
- Only Living Flesh, "...something about the field generated by a living organism...." can be transported via the "Temporal Bubble"
- The TDE requires obscene, huge amounts of power to function.
Whatever mechanism of physics allows this "Temporal Bubble" to be selective to living tissue reacts to non-living tissue in a violent manner, displacing or disintegrating the matter that would exist within the boundaries of the "Temporal Bubble" at its destination, leaving the matter directly outside of the bubble in an excited state - a state that displays the glowing properties noticed in the time travel scenes. The nature of this state is not determinable, as its effects are only observable.
Due to this effect, we see sections of matter, like the cut-open truck and the concave impression in the ground during the events of Terminator 1, glowing - as if gouged out by a sphere shaped implement.
This glowing effect could be a phosphorescent or light based effect and not a heat effect - it may be only perceived as a heat effect due to the reddish coloration of the visual effects used.
The "Temporal Bubble" is associated with lightning-like effects while being observed, and these elements may be the cause of any fires seen during "Temporal Bubble" scenes, as opposed to a result of the glowing effect.
If the glowing effect is, in fact, indicative of a high temperature, the living tissue of an T-series Infiltration Unit would only be in contact with this for a short duration, and as seen by fire-walking humans, this can be withstood by human flesh for a short duration of time. This assumes that Skynet engineered Terminator flesh has the same exact properties as regular human flesh, and I submit that Skynet engineered it to be better than regular human flesh, therefore higher durability is likely.
A T-series Infiltration unit is designed to mimic human appearance enough to get close to a target while being resistant to damage - something so insignificant as a third degree burn on the feet is not going to be a consideration for the machine, especially since the injury can be covered up by any footwear of appropriate size the infiltration unit can acquire. As demonstrated in several scenes, a T-series infiltration unit is very good at acquiring inventory for a mission.
Skynet is a machine intelligence that is constantly trying to improve its equipment and technology, and I propose that the differences in the effects of the "Temporal Bubble" leave upon its destination are the result of this improvement cycle - leaving less evidence of a "Temporal Bubble" is advantageous as it retains the element of surprise, not giving those sneaky humans any evidence that an infiltration unit, flesh covered weapons pod, mimetic polyalloy, or nanomachine colony has been sent to the target area.
In canon, power generation was always an issue for Skynet, who still competed with humans for resources - a TDE device which wasted less power during temporal insertion, as observed by less of an effect to the target area, would be advantageous in several ways. More temporal insertions could be accomplished with less power, allowing that energy to be put into interdimensional transportation or timeline-altering research processing.