Not possible to use buttons in panel header by default. In material theme button background color is transparent and text color matches header background color. Which means you don't know there is a button there unless you hover/click it.

https://fiddle.sencha.com/#view/editor&fiddle/2vjc

Code:
Ext.application({
    name: 'Fiddle',


    launch: function () {
        Ext.create('Ext.panel.Panel', {
            title : 'Some title',
            html : 'Some content....',
            header: {
                items: [{
                    xtype: 'button',
                    text: 'Button which is invisible in Material theme'
                }]
            },
            renderTo: Ext.getBody()
        })
    }
});
Normal view:
Снимок экрана 2019-09-23 в 20.45.26.jpg
On hover:
Снимок экрана 2019-09-23 в 20.45.36.jpg
On click:
Снимок экрана 2019-09-23 в 20.45.43.jpg

P.S. Fixed locally with the following sass:
Code:
.#{$prefix}panel-header .#{$prefix}btn {
    background-color : rgba(255, 255, 255, 0.1);

    .#{$prefix}btn-icon-el,
    .#{$prefix}btn-inner {
        color : #fff;
    }

    &.#{$prefix}btn-over {
        background-color : rgba(255, 255, 255, 0.3);

        .#{$prefix}btn-icon-el,
        .#{$prefix}btn-inner {
            color : #fff;
        }
    }
}