Half notes or tied quarters?

Come on, Dorico, make your mind up?
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I know there is a global setting for which Dorico uses, but is it possible to override the setting in individual cases? If not, please make that a high priority for the next release because it’s driving me crazy!

I presume it’s because one is preceded by a rest an the other is preceded by a note.

And yes, you can always use Force Duration (O) to override the notation however you wish, once you’ve fiddled sufficiently with Notation Options.

For more information about Force Duration, see here.

As Dan says, the reason you have tied quarter notes in the up-stem voice is because of the rest at the start of the bar. The down-stem voice has a note at the start of the bar, so Dorico’s syncopation detector considers that a valid short-long pattern and thus allows the half note to be notated as a single notehead even though it crosses the half bar.