I compromised by going back to the way it was and calling the *separate* script using <EXEC> rather than <EXEC BKGND>. Again, the results are the same. The script, when called with <EXEC> returns different results, even with the BKGND option.
(Granted, this may not be a definitive test, since it's back to being two separate scripts. But they look to be identical.)
Everything is all on the same server, and there is only one version of Texis installed on it, so no... the versions are the same.
As for which which result set is correct... Well, there seems to be issues with *both* result sets, frankly. I suspect that this stems from whatever issue is causing the anomalous results described in [Tkt#4ea85e0ab]. The query here has all the same characteristics as those described in that open issue. (It's crossed my mind that all of this is part and parcel of the same underlying issue, but I can't say for sure.) The foreground search returns 2195 hits while the background search returns 2199 hits. Moreover, ALL of the hits from the foreground search are included in the background search results. That is, there are four distinct additional hits returned by the background search. But here's where things get a little weird... Of those four, three look like they should NOT be hits. One of them, however, looks to be a legitimate hit, and yet is NOT returned by the foreground search. As I said before, the queries are big and complex, the fields being searched contain a lot of data per record, and there are many records. Which is all to say that we don't yet know if there other problems with either result set.
A CGI texis.exe lives in your web server's CGI directory, which might have been moved around since the original install and would cause an upgrade to not change that file. You should be able to copy your installationdir's texis.exe over the CGI one to bring them both back in line.
Mystery solved. So, to all who helped me on this... Yes, my mistake, and I apologize for all head-scratching this caused ... Some time back, in an effort to streamline our environment, I changed the CGI directory to something other than the default. During an upgrade of Texis, however, I suppose failed to indicate to the installer what that new directory was, and from that point on we've been running different versions, depending on how Texis is called.