The second is not surprising, given that the Wikipedia dataset (Hwang et al., 2015) does not allow for one-to-many sentence alignments and therefore does not contain sentence splitting examples.
when trained on a dataset that contains examples of sentence splittings (the Newsela dataset), our NTS models were able to learn this simplification operation and successfully apply it on a test set from either the same domain (examples 2a-2c, Table 8) or from another domain (examples 5a-5b, Table 8).