So apparently, I'm not the only person having this problem, so it's a known bug. When can we expect a fix for it?
Now here's what's truly aggravating: the problem is specific to 12.x Mac. I spent the weekend verifying that the problem does NOT occur in the Windows version of 12.x, OR in VWA 10.0, 11.0, 11.5 Mac in either Panther (10.3.9) Tiger (10.4.0, 10.4.2, 10.4.6, 10.4.8, 10.4.9, 10.4.11) or Panther, and occurs on all three Macs I tried it on.
The problem is NOT permissions-related: it occurs for every possible combination of file and folder permissions for Owner, Group, and Guest. It is also NOT due to a third-party sw conflict, since it occurs on a fresh system installation consisting of ONLY the default Tiger Retail installation and VW Fundamentals+Architect.