Undo doesn't work after extending barlines if the last instrument is hidden

While I was translating the Lesson 1 on transiflex, I tried to extend the barline to join two staffs as presented in the lesson, on a score that I was working on. I came across a really strange effect when I wanted to undo the modification.

- Open this score
- select (double-click) one the barlines of the above staff
- extend it to the lower staff
- "accept" it (by clicking somewhere else). Now all the barlines are connected between the two staffs (as expected).
- now Undo (Ctrl-Z)
---> the separated bars are not restored :-( (but the dirty flag (star) is removed).

I've striped the score of everything to find out what was leading to the problem: the score has a hidden wood-blocks staff (that was used as metronome) as last instrument on the list.

I've tried a few more things:
- if I make the wood-blocks staff visible, undoing the barlines extension works as expected.
- If I keep the wood-blocks staff invisible but move it between both saxs, undoing works as expected.
- it's not specific to the wood-blocks. If I remove the woodblocks and add for example an invisible trumpet as the last/lowest instrument, I have the same effect, undoing doesn't work.

One last point: according to the inspector, the span preset of the barline is modified in the error case, after the barline extension:
- if the last instrument is visible, it stays on "stave default"
- if the last instrument is invisible, it becomes "custom".

(Win7 - version 2.0)