TLDR: the problem was mistake in an Apache mod_rewrite directive; it had nothing to do with Asset Mapper.

As noted in one of my comments above: it was because of an Apache mod_rewrite condition in my vhost config, which needed to be either inside a Directory block or else be written as `RewriteCond %{DOCUMENT_ROOT}%{REQUEST_FILENAME} !-f` , i.e., including the %{DOCUMENT_ROOT}. The test was coming back false, so files that did exist were not getting served. As for the issue with the dev vs prod, I'm not sure it was a total illusion, but it led me down the wrong path (so to speak). Of course, this whole question I've posted is such an embarrasment, I'm tempted to remove it. But there is a chance someday it could help somebody.