Basicly GL screensavers with a plain black background are transparent, others work.
If you look at gl test and set the rendering in it's extended settings to e.g. grey, it works.
With GL, StaticGrey and True Color it fails.
Not sure if that is a xfce bug though.
Maybe related to http://bugzilla.xfce.org/show_bug.cgi?id=3648
I cannot reproduce, can you elaborate on the reproducer?
Also, what hardware/driver are you using?
I don't have access to that box right now. Can check on Monday. I think it was fbdev on virtualbox.
I can't reproduce here too.
Driver is fbdev. I can't reproduce it with vesa.