Hi again!
I found that method store.findExact() call store.data.findIndexBy();
For most stores store.data property is Ext.util.MixedCollection which has findIndexBy() method.
But if store.buffered data property is PageMap (extend: 'Ext.util.LruCache', 'Ext.util.HashMap'). All this classes has no findIndexBy method =(

Code:
       if (me.buffered) {
            me.data = new me.PageMap({
                store: me,
                pageSize: me.pageSize,
                maxSize: me.purgePageCount,
                listeners: {
                    // Whenever PageMap gets cleared, it means we re no longer interested in 
                    // any outstanding page prefetches, so cancel tham all
                    clear: me.onPageMapClear,
                    scope: me
                }
            });
            me.pageRequests = {};

            me.sortOnLoad = false;
            me.filterOnLoad = false;
        } else {
    
            me.data = new Ext.util.MixedCollection(false, Ext.data.Store.recordIdFn);
            me.data.pageSize = me.pageSize;
        }