It seems to me that when you add the dashed barline, you split the bar in two with a hidden time signature in the second half. The first half is now an incomplete bar. With incomplete bars, Dorico seems to remove the beginning of the bar rather than the end (this is logical for pick-up bars). Therefor - the two first notes are actually at the end of the bar.
As you can see in the attached screenshot, 3+4/4 gives you what you want, because you have specified that the end of the bar should be grouped in 4.
As shown in example C, another way to achieve what you want is by specifying alternating meters (write “4/4 + 3/4” in the popover). If you change the middle barline from solid to dashed now, Dorico maintains the alternating meter property.
To make everything look and behave as you want, I’d fix the first bar using the force (pun intended) and add a hidden 4/4+3/4 meter thereafter.