Per ValveSoftware/Source-1-Games#6868 loading packed files from BSPs (& VPKs!) has some casing issue that popped up recently for Linux users.

The problem stems from CZipPackFile::GetFileInfo() (and CPackFile?) which is used when trying to open packed files.

Previously (at least in CS:S previous_build) it would do something like this:

CPackFileEntry entry;
entry.hash = HasStringCaselessConventional(filename);
int i = this->files.Find(entry);
if (i != -1)
{
	// return file info here...
	return 1;
}
return 0;

Now it looks like it does something like this:

CPackFileEntry entry;
entry.hash = HasStringCaselessConventional(filename);
int i = this->files.Find(entry);
if (i != -1)
{
	FileNameHandle_t f = this->filesystem_maybe->FindFileName_maybe(filename);
	if (this->files[i].handle == f)
	{
		// return file info here...
		return 1;
	}
}
return 0;

This is making its way to CUtilFilenameSymbolTable::FindFileName()

So even if the packed file did find a match case-insensitively, the FindFileName() causes it to fail.

(Same with CUtilFilenameSymbolTable::FindOrAddFileName())

TL;DR: Remove the #ifdef _WIN32 wrapper around Q_strlower( fn ); in CUtlFilenameSymbolTable::FindFileName and CUtlFilenameSymbolTable::FindOrAddFileName.