RemoveEmptyStaffContext and merge rests snippet not co-operating

[Frank Steinmetzger]

Dear list

Using 2.14.2, I am setting an SA-TB choir score with an additional solo voice.
With the help of snippet 336¹ I am auto-merging rests in the two-voiced
staves.  I also want to use \layout{ \context{ \RemoveEmptyStaffContext } }
in order to hide the solo staff where it is silent. 

While the empty staves get removed as expected, using this feature breaks the
merge-rests code -- I get two rests above each other.  Can you per-chance tell
by looking at the snippet with your trained eye what could fix this (if at all
possible)?  It’s not really a showstopper, I can do it the old-fashioned way:
\oneVoice r4 \voiceOne

I’d be just more convenient if I wouldn’t have to. :-)
Thanks and good night...

¹ http://lsr.dsi.unimi.it/LSR/Item?id=336
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