I use picker on my phone, so I cannot multiselect;I tried it on my computer, it works, maybe picker api is not for mobile --- There is a pending Issue Tracker Post which is a bug related to this post. I suggest hitting the +1 button to signify that you also have the same issue and consider adding a star (on the top left) for Google developers to prioritize the issue.

According to Google's end comment (#3) from [email protected]<[email protected]> :

I have reported this to the engineering team and future updates will be shared here. Thank you for your patience.

Reference: Issue Tracker