In captivity they can be considered 100% aquatic.
If an animal is ill, stressed, bullied, or there´s not enough water, or not good enough, or the temps are wrong, then it might go onto land to avoid said problems.
How big is the tank where you are keeping them? And what temp are they?
I don´t know if you are aware of this, but all Paramesotriton sold in pet-shops are wild caught. The importation process is usually hard on the animals and those who survive still have to deal with the pet-shop, where they usually receive very poor care. This all means that newly acquired animals will be stressed and possibly ill. In orther to avoid illnesses (brought up by a decreased inmune system), and allow the animals to recuperate from everything, you need to provide excelent housing and water conditions. This is VERY important, so the first thing to do is to make absolutely sure that you are meeting all their requirements.