The documentation of the option "pattern" is wrong. It states:

This option is only relevant when the widget is set to choice. The default pattern is based off the format option, and tries to match the characters M, d, and y in the format pattern. If no match is found, the default is the string {{ year }}-{{ month }}-{{ day }}. Tokens for this option include:

This is not the case. The pattern of the displayed fields is controlled by the option "format", and by "format" only.