After more investigation I find that this bug occurs for me if using Brasero as a stand-alone programme, or if I try to use it from Nautilus in a tabbed window.
In more detail; if I have two Nautilus windows open, one for the CD burner (Brasero) and the other for the source files, then attempting to copy more than a single CD of data *does* function correctly, with the data files spanned over as many CDs as are needed.
On the other hand, if I have a single Nautilus window open, with two tabs, one for the source files and one for the CD burner and then drag the files between tabs I get the bug that I reported above.
After more investigation I find that this bug occurs for me if using Brasero as a stand-alone programme, or if I try to use it from Nautilus in a tabbed window.
In more detail; if I have two Nautilus windows open, one for the CD burner (Brasero) and the other for the source files, then attempting to copy more than a single CD of data *does* function correctly, with the data files spanned over as many CDs as are needed.
On the other hand, if I have a single Nautilus window open, with two tabs, one for the source files and one for the CD burner and then drag the files between tabs I get the bug that I reported above.