When I select several slides in the order of service, Click on Background > Browse Media >
Select a background and then click "Apply as Background", it only applies the background to the last slide selected.
If I use "Apply as Smart Media", all the slide backgrounds are changed, but if the slide has a text box with "Box fill Justification" sometimes the text of the slide is changed as well.
See the two Intercessory slides in my presentation for 2020 Jun 7
I am pretty sure that changing multiple backgrounds did work before.