While the experimental protocol involved in this research wouldn't pass most ethics committees, however if we assume aliens are ethically challenged in the way they conduct research, then this is plausible.
It's all a matter of considering what sort of research is being conducted in these experiments. Basically we know humans are being kidnapped and transported to a terraformed planet. This process has been going since prehistoric times. So humans will have developed, initially, a palaeolithic culture. Presumably as things go on, their technology and culture will develop and various forms of civilization will arise.
However, over time the civilization(s) on the terraformed planet will be supplemented by the continual arrival of more humans who will bring with them the latest developments of technology and culture from planet Earth. Possibly the humans on the experimental planet will have learned to expect this continued influx of new arrivals and adapted their cultures accordingly.
Now we come to the crux of the matter. What is the nature of the experiment being run by the aliens? This by now should be obvious. It is an experiment in continuous adaptation and development of members of a sapient species, in this case humans, who have been transplanted in the environment(s) of a different world, in this case a terraformed planet.
While it is not unreasonable to condemn this experiment on ethical grounds. It is, after all, being conducted without the consent of the experimental subjects and their descendants are forced to remain as inhabitants of the terraformed planet without any redress. This type of research does appear to be quite plausible and could be carried by almost highly advanced technological civilization where its scientific practices are more than somewhat ethically challenged.
In conclusion, understand the science and you can understand the reasons for doing it. This makes it plausible. Not nice, but plausible.