(Pixelmator Pro 1.0.9 on macOS 10.13.4 on MacBook (Retina, 12-inch, Early 2015))
I know that there are a few group bugs you are busy squashing at the moment so it's possible this is already being sorted.
If not, here's how to reproduce.
First, let's see it working.
1. Create a new document.
2. Add text.
3. Duplicate the text object.
4. Group the two text objects.
5. With the group closed and selected, grab the handles and with the ⌘ key pressed, rotate the group.
The two objects should stay together.
6. Open up the group again and change the style of one of the text objects.
7. With the group closed and selected, grab the handles and with the ⌘ key pressed, rotate the group.
One object rotates and the other orbits around it without rotating.
I hope you can reproduce.
Is something not working like it should? Let us know.