Google throws this exception on Activity's onCreate
method after v27, their meaning is : if an Activity is translucent or floating, its orientation should be relied on parent(background) Activity, can't make decision on itself.
Even if you remove android:screenOrientation="portrait"
from the floating or translucent Activity but fix orientation on its parent(background) Activity, it is still fixed by the parent, I have tested already.
One special situation : if you make translucent on a launcher Activity, it has't parent(background), so always rotate with device. Want to fix it, you have to take another way to replace <item name="android:windowIsTranslucent">true</item>
style.