TagField is broken, I see that the setValue method of TagField has been changed and now it is broken for single select elements, see fiddle:

https://fiddle.sencha.com/#fiddle/vet

The problem is in this piece of code in the setValue:

Code:
if (Ext.isEmpty(value)) {            value = null;
        } else if (Ext.isString(value) && me.multiSelect) {
            value = value.split(me.delimiter);
        } else {
            value = Ext.Array.from(value, true);
        }
Before it was:

Code:
if (Ext.isEmpty(value)) {
            value = null;
        } else if (Ext.isString(value) && me.multiSelect) {
            value = value.split(me.delimiter);
        }
        value = Ext.Array.from(value, true);
Then value was never null. Later in this method there is a check:

Code:
// For single-select boxes, use the last good (formal record) value if possible
        if (!me.multiSelect && value.length > 0) {
That breaks because value is null.

Together with this other bug report I posted time ago makes tag field unusable