Counterflashing
Metal cut into the mortar joint has to turn out over the step flashing. If it has lifted, rain runs behind it.
Cerritos chimneys on tile roofs leak at the counterflashing, the apron on the downslope side, and the cricket where debris sits. We repair that roof joint. Rebuilding a cracked brick stack is masonry work. If the brick itself is the problem, we say so instead of coating the roof around it.
People search chimney leak repair, leak around chimney when it rains, and fireplace ceiling stain.

Metal cut into the mortar joint has to turn out over the step flashing. If it has lifted, rain runs behind it.
The saddle behind a wide chimney keeps water from pooling. A flat pocket fills with leaves and leaks.
A crown that dumps water onto the flashing is a different fix than a hole in the tile.