I’ve needed to move notes from one location to another and I’ve noticed this happening.
I had to bring the beginning of the notes from the previous half and Dorico first moves the notes forward (note bar in brackets) and then changes the tempo (brings it to 2/2) automatically.
Please check, because if I remember correctly, it happened with version 4.xx as well.
By dragging a rectangle around those bars, you also selected the boxed bar numbers. These are generated by the time signature. So when you Paste, you are pasting the meter and the notes in relative rhythmic position to that meter!
A better way to select the passage is to click on the first bar (or the first note) and Shift-click on the last bar (or note). This avoids selecting any system objects such as rehearsal marks and bar numbers. Then instead of Cut & Paste you can move the music back by the rhythmic grid with Alt-left arrow.
Or, even easier, you could turn on insert mode, select the errant half rest, and delete. (Turn off insert mode afterward.)