I've had this problem for awhile but in 2.16.0 hit on something that worked once. Whether it can be generalized to a _solution_ I don't know. Here's a snippet, I hope it helps (it's from a cadenza-like passage in a violin part of a quartet (composed ca.1899, published posthumously 1904), f.w.i.w. ...)
\stemNeutral g8[( \times 4/6 {\stemUp a!32 b c d e f)]} \stemNeutral \afterGrace \pitchedTrill g2(\fermata\startTrillSpan as { fis16[ g]) bes4^\markup{\italic"rit."}^^ as^^ g(^^ e!\stopTrillSpan)^^}