I've had an off-list exchange of e-mails with Jonathan Rogness, who reports that this may be resolved by setting singleResult => 0, instead of singleResult => 1. This suggests that there is something about how the MultiAnswer checker chooses to define its answer blanks that's causing the problem (which makes very good sense).
This is really a work-around rather than a solution, but it's a start!
Gavin
WeBWorK Problems
parserMultiAnswer and Tests
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