Sometimes a bit of mortar will seep through the mortar joints, but it will have absolutely zero effect on the performance of the oven.
Some customers want to grind down, chisel off or break off that mortar - DON’T DO IT!
First, as mentioned before, a little bit of mortar showing will not effect the performance of the oven. Second, the entire inside of your oven will be black (fire / soot) by the time your curing fires are complete. And third (the most important) it you break, chisel or grind the mortar off, you risk cracking the mortar joint.