Not sure what exactly is messing!
I have tried filling a color your rectangle, however, was unable to do so.
Then I tried creating my own rectangle and applying the drop shadow to it, but it wasn't applying.
Will test it out tomorrow morning.
Definitely, something is corrupted in that page, as the same work well in a newly created file.
So not sure probably may try creating a new page, copy the other objects from the previous page and recreate the defective objects.
I´ll give it a try with a new page and let you know.
Not sure though, why it didn`t work on master, which is kind of a new page, isn`t it?
Best Regards and many thanks for watching out,