This is just one manifestation of the way "bars are constructed in printing in BIAB. All bars on a particular line, regardless of the number of notes or lyrics in the bar, are the same physical length. A bar containing a whole note, 4 quarter notes or even 1 eighth note has the same length as every other bar on the staff.

So the bar with your eighth note lead in is the same length as every other bar on the first line.

It has been the same FOREVER. I don't see any work around other than perhaps not printing the lead in bar even though it does have a note in it. It will mess up the count in somewhat, and you could always use the well documented pencil addition.

But I think it would be better to have BIAB dynamically allocate the bar length based on the number of notes or amount of lyric associated with the bar. No more fixed bar lengths.

As this is unlikely to happen anytime soon, perhaps we might find a way for music XML to transfer all of our music to a good print program. (I wish)

BIAB for Windows Version 2019 (643)
Windows 10 (1909)