Welcome to the forum!
Have you tried with multiple symbol sets/files? One possibility is that you got hold of a symbol file that isn't made right. Another possibility is that it wasn't installed correctly.
Try using the symbol sets that came packed with Inkscape (if you haven't already) and let us know what happens.
As far as I understand, even with the special configurations that are needed for using Inkscapes on Macs, you still need to avoid pasting.
I know you mentioned several things you looked at, to rule out transparency. But did you see this? It covers ALL possibilities for transparency issues. So you might review that, to make sure you checked every place. They all aren't relevant for symbols, but just to be thorough.
I found one bug about using symbol sets that come from Visio:
https://bugs.launchpad.net/inkscape/+bug/1662465 Didn't find any other bugs that sound similar. So if we can't figure out the problem, we may need to file a bug report.
However, while reading through bug reports, I had one more thought. Have you tried using symbols in different documents, or have you only tried it in one particular file? If you've only tried in one file, does it have other content in it? If you've only tried in one particular file, try with a new blank doc.
Are you using any special template, or just the default? Have you made a custom default doc? If yes to either, try using the original default document, with no template. If that seems to resolve the problem, we'll need to investigate the template or custom default.
Sorry for all the questions. This is a problem I haven't heard before, so just thinking through everything.
