MP3s created with --nogap have encoder delay 576 in the first file, 0 in the rest, and only the last file has padding. If --nogaptags is used (e.g. to assist with playback of the files individually), then an identical LAME tag is written to each file, reporting encoder delay 576 and whatever the last file's padding is. The LAME tags should instead accurately reflect each file's content: 1st file has delay 576, padding 0; intermediate files have delay 0, padding 0; and the last last file has delay 0, padding whatever.
Decoder behavior when the padding is less than the decoder delay is unpredictable, and separately-decoded "nogap" files are unlikely to be seamless when played end-to-end, but we should at least write accurate tags so as not to compound the problem. Inform the decoder front-ends that the delay and padding is what it is, and let them deal with it.