Windows 10 home, Illustrator 2020.

After uniting two or more rectangles on the perspective grid to make a more complex shape, moving/copying the new object using the perspective selection tool results in distortion as the object appears to have become detached from it's perspective plane. The behaviour is shown in the attached image.

Inserting the step Object > Perspective > Attach to active directly after Unite allows the object to move correctly (thanks Monika Gause) although it was suggested that perhaps this step shouldn't be necessary and it should be able to move correctly without it.

Many thanks, Craig.