This is probably a bug with the same root cause as bug 2953949, but I add a report for it still since I believe this variation to produce the error give more information about the issue.
The EBNF production
ConditionalExpression ::= ConditionalOrExpression ( "?" Expression ":" Expression )?
gives an picture with an erronous extra mandatory Expression at the end.
The thing to observe is that it is not the whole optional part that is repeated, but just the last Expression. Hopefully this information can make it more simple to track down the problem.