Hello again, I haven't been able to solve the problem yet, but I did a little bit of testing. It seems that the issue has something to do with multiple voices; however, I cannot really pinpoint it.
The issue in my sample code is resolvable by placing more than a few notes inside parts with multiple voices. Replacing the vOssiaRight staff with \relative c''' { \vGlobal << { \ottava #1 f8-!\noBeam \ottava #0 a,,32_( g f e) } \\ { a'8 s } >> d,,8-!\noBeam \ottava #1 <f'' a f'>8-! | <d b' d>-!\noBeam \ottava #0 d,,32( c b a) g8-! \ottava #1 << { g'''32( f e d) } \\ { g,8 } >> | << { e'8-!\noBeam \ottava #0 g,,32_( f e d) } \\ { g'8 s } >> c,,8-!\noBeam \ottava #1 e''32( g c e) | \break fixes the barline. These two bars and one other bar are the only instances where there is a single-note multivoice part in either of the ossia staves. However, after replacing them with this partial workaround, the final barline continues to be an issue. I wonder what's going on here... Cheers, Hans