I can't seem to reproduce this issue here, eg. with the following code both 6.16 and 6.17 display the same thing here:

tabtest {
window -aln -t2,3,6,8,10 @test
aline @test aaaaaaaaaa $chr(9) bbbbbbbbbb $chr(9) cccccccccc $chr(9) dddddddddd $chr(9) eeeeeeeeee $chr(9) ffffffffff
}

Is the problem only happening when you use certain fonts, or certain combinations of tabs and text? Can you provide an example that I can use to reproduce the issue?