From a thread on electro-music forum:

“The CD4011 is a bit fussy … a few years ago I repaired one of these machines with a bad 4011, and the replacement I used caused pattern writing to work incorrectly. A different 4011 fixed the problem.”

“I had the same problem for long time until have found the solution.
Is easy to fix it changing C46 to 10NF cap, an will work with any 4011 series.
In the service manual they say “machines with SN up to 923500 CD40UB only”. this is because the propagation delay time of this chip is faster than other 4011s (a half) and the delay formed with Q16 C46 etc is not enough to feed the flip flop and the memory on time (must be double).

I came across this extremely useful piece of information while researching this project.

Many thanks to electro-music forum for this one!