Why are leader skips seemingly ignored in vsplit?

I’ve gotten an expected result with a vplit on a box with leaders. The leaders have a fixed amount of glue, plus stretch and shrink. The fixed amount seems to be ignored in the size calculation, or else TeX is shrinking to get more in the split box than it needs to. Consider:

setbox1=vbox{
 vbox to 50pt{}
 nobreak
 leaders hbox{} vskip25pt plus 100pt minus 25pt
 allowbreak
 nointerlineskip
 vbox to 50pt{}
 nobreak
 leaders hbox{} vskip25pt plus 100pt minus 25pt
 allowbreak
 nointerlineskip
 vbox to 50pt{}
 nobreak
 leaders hbox{} vskip25pt plus 100pt minus 25pt
 allowbreak}

Then:

setbox3=vsplit1 to 150pt

showbox3
> box3=
vbox(150.0+0.0)x0.0, glue set - 1.0
.vbox(50.0+0.0)x0.0
.penalty 10000
.leaders 25.0 plus 100.0 minus 25.0
..hbox(0.0+0.0)x0.0
.penalty 0
.vbox(50.0+0.0)x0.0
.penalty 10000
.leaders 25.0 plus 100.0 minus 25.0
..hbox(0.0+0.0)x0.0
.penalty 0
.vbox(50.0+0.0)x0.0
.penalty 10000
.leaders 25.0 plus 100.0 minus 25.0
..hbox(0.0+0.0)x0.0
.penalty 0

As you can see, TeX puts all three of the inner boxes in box3, although their natural height, including the leaders is 225pt. There’s plenty of stretch to include just two of the inner boxes and get a box whose height is 150pt. So my question is: why isn’t TeX respecting the leader‘s fixed amount of glue in the above example?

(I’m not aware of a tracingsplits, analogous to tracingpages, which shows the calculation.)