Why Tied Quarters?

Why is Dorico entering a pair of Tied Quarter notes instead of a half note, which is what is selected.
Tied Quarters.png

Because your Notation Options > Note Grouping is set to do so. Go in and change it.

Dorico doesn’t recognise that as a default short-long-short syncopated pattern because the short notes need to be single notes, i.e. quarters, rather than pairs of notes.