Content assist does not work for common prefix rules

[cdietrich]

given the grammar

entry Model:
    (persons+=Person | greetings+=Greeting)*;

Person:
    'person' name=ID;

Greeting:
    GreetingByName | GreetingByAlias;

GreetingByName: 
    'hello' person=[Person:ID];

GreetingByAlias:
    'hello' alias=ID (':' data=PersonRef ('{'

    '}')?);

PersonRef:
    person=[Person:ID];

content assist does not work at

person Aaaaa

hello a : |

not at

person Aaaaa

hello a : A|