Problem aligning colon with optional parameters (question mark)

[canuda10]

in a case like:

private a: number;
private another: number;
private explicit!: number;
private optional?: number;

with the configuration "colon": [-1,1] I expect the code to be formatted like:

private a:         number;
private another:   number;
private explicit!: number;
private optional?: number;

However, it is formatted like:

private a :         number;
private another :   number;
private explicit! : number;
private optional?:  number;

adding one empty space on all parameters except the optional one...