Decrease from DT=38 to DT=30 within 12 hours is way too high. Normally, it should be down to 37 or 36 in 12 hours, depending on the strings. I can think only about 3 possible reasons:
1) Racquet was stored for several hours in a very hot environment (e.g., in a locked car left in the sun or in a car trunk). When strings heat up, the strings material becomes softer, this makes it easier for them to creep [this is a technical term for mechanical deformation of plastics]. Consequently, strings elongate and lose tension significantly.
2) The string is unusually "stretchy" and must be pre-stretched before or during stringing. If not pre-stretched, such material would creep quickly and significantly within the first few hours. Cheaper strings are more likely to show such behavior. Slower pulling speed on a constant pull machine could largely compensate for it. One can also use the prestretching function on the machine [pull at a higher tension, release, pull at the target tension].
3) String was somehow defective from the factory (not very likely, but theoretically possible).
Also, I do not feel like adding this to the list of possibilities, but DT strongly depends on the position on the racquet and increases from center towards the frame. It is highly unlikely that you would overlook this, but I will mention it anyways: it is important to place the ERT-300 exactly in the center of the racquet each time you measure, to get comparable results in two measurements.
Finally, even less likely possibility... if the string is natural gut and it got wet, it is more likely to rapidly loose tension under softening effect of moisture on the gut.
