I guess all this was intended. First of all it's such an important thing, if it was a bug it would be fixed a long time ago (and Zanz would not increase reward to 5 befor fixing it), so I'm 99,99% sure it's all intended.
On the other hand I don't know why only first two cards matching give the electrum reward... maybe it has something to do with how real slot machines in casinos work.