Could the attached case be fixed in future version of Dorico?
You can fix it yourself by using the voices correctly - the half rest is screwing up the slur and is in the wrong place. Presumably the red voice is actually a downstem voice that you’ve flipped upwards, or the half note rest would be above the staff.
I get this with Dorico 3.5 default settings:
But if you’re determined to have the half rest under the staff, all you need to do is set the slur’s Disable auto curve adjustment property in Engrave mode.
All that said, should your example really be in three voices?
Thank you for your concrete and detailed explanation.
Indeed, it is only two voices. I converted it to three voices due to the melody line often lower or higher than the whole notes.
You are correct; I can allocate notes correctly to voices.