I reported this to Support and they gave an override to resolve it. Giving it back to the community here:

"Combobox had been using displayTpl for the chosen value and the list of options. Now in 7.1 it's only used for the list of options. Same for 7.2."

Related fiddle: https://fiddle.sencha.com/#fiddle/353f

Solution as an override:
Ext.define(null, {
    override: 'Ext.form.field.ComboBox',

    updateDisplayField: function(displayField) {
        // force auto generated displayTpl refresh when displayField changes
        if (displayField && this.displayTpl && this.displayTpl.auto) {