After conducting a "controlled experiment" and studying log files, I found the cause of this behaviour: Once the import of one AVI fails, the program no longer calls the correct analyser, so any subsequent attempt during the current session will fail. My experiment was to import the following files:
E:\Test\1 350 MB video.avi
E:\Test\2 Null file.avi
E:\Test\3 350 MB video.avi
The first and third files are normal AVI videos which have been previously imported (with different names) without incident. The second file is a null file which is expected to "break" the scanner so it will fail to properly import the third file.
As expected, the first file imported correctly. According to the log (and making a long story short), the program calls JRWorker.exe with the /AnalyzeDX parameter to do so.
For the second file, of course, this fails. It tries again with a /AnalyzeRP parameter (apparently something to do with a "RealPlayer engine"). This too fails, and the file is therefore added as a Data type.
Here is where the problem is: When the third file is processed, the program calls JRWorker.exe with /AnalyzeRP, instead of the /AnalyzeDX parameter.
The second part of the test was to change the name of E:\Test\1 350 MB video.avi to E:\Test\4 350 MB video.avi, delete the other two files, and import the folder again. The result was the same—the /AnalyzeRP parameter was used immediately, and the file was imported as Data.
Finally, I shut down MC, changed the name of E:\Test\4 350 MB video.avi to E:\Test\5 350 MB video.avi, restarted MC, and imported the folder again. This time, the file was properly analysed using the /AnalyzeDX parameter, and the file was correctly imported.