I guess this is down to permissions on the System module, which you can't change. One option is to create a new attribute for LastLogin_date in your Account object (derived from User), and set the value to this field through a microflow from the User record. You can then set whatever permissions you need on this copy field.
I've run into this before but it indeed has come down to security reasons. You may want to check the reference path to that attribute, if account is not your form's main entity then you need to make sure you grant permission over the association. You may want to check and be sure that you've assigned the proper attribute to the box. Also, may seem like a silly question but did you redeploy your application?