Dorico is simply trying to apply its notation rules for 3/4 bars by clearly identifying where each of the beats fall, hence it creates the ties across the beat boundaries.
You either need to force duration, or get Dorico to apply different rules by using a hidden time signature.
What you are actually asking for here is the notation for a 12/16 bar, beamed as 6+6 (which you could enter as shift-M [6+6]/16). You can hide this using the properties panel.
You can choose which route to go, but it likely depends on what is happening in other parts at the same time.
For a bar like this, if the prevailing time signature is 3/4, I would insert a hidden 6/8 time signature. It would get you the beaming you want right away.