\version "2.19.25" \new Voice << { c''^~ c'' } { a'_~ a' } >> \new Voice << { <c''^~> c'' } { <a'_~> a' } >> { <c''^~ a'_~> <c'' a'> }
Contrary to (at least my) expectation the first example gives
tie-within-chord.ly:6:33: warning: Two simultaneous tie events, junking this one \new Voice << { c''^~ c'' } { a' _~ a' } >>
and applies the first tie to both notes.
The other two give correct output, and it would serve consistency and predictability if the first did also.
Issue 2240 was concerned with a similar topic.
Admittedly, Type:Defect is a bit of an overstatement, but Type:Enhancement would be understated again/wouldn’t fit the problem, since this isn’t about adding functionality, but about adding consistency.