Reordering instruments within barline spans creates "custom" barlines that won't extend properly

Ubuntu 14.04, GIT commit: c0bb98b

1) new score, flute + oboe
2) extend barlines through score - all barlines extend, as they should
3) edit / instruments
4) move oboe up (above flute)
5) OK - barlines are reset, which is maybe not ideal but not a big problem in itself
6) extend barline through score

Result: only the current barline extends. Apparently, all the barlines in both sets have been set to "custom", and this is never cleared until you save/reload.

I discovered this while working on #68806: Bad barline spans with one-line staves after hiding / removing spanned staves.