Why doesn't the Catholic Church view Plymouth Brethren baptisms as valid?

The Catholic Church seems to [accept most mainline Protestant baptism as valid](https://www.dosp.org/wp-content/uploads/9_3.-Valid-and-Invalid-Baptisms.pdf) , but does not recognise the baptisms of non-Trinitarian denominations. There are a few Protestant (and Trinitarian) denominations also in the invalid list however, one of which is the Plymouth Brethren. This is surprising to me. Are there some theological problems in the Plymouth Brethren's beliefs, either about the Trinity or in the baptism formula they use, that cause their baptisms to be considered invalid in the eyes of the Catholic Church?