Oh hey I found a bug, let me file a bug report. Oh, it already exists. Opened 9 years ago. With 27 comments. And two dupes. *sigh* Here's a diagnosis of the problem. And here's a patch to fix it, because clearly if I don't nobody else will.https://bugs.launchpad.net/inkscape/+bug/381677 …
Same issue, with blur involved? FWIW the workaround, for less complex objects anyway, is to apply transparency as the alpha color component in the individual objects instead of using object/group opacity. Annoying but doable in many cases.