I'm probably being over cautious (it's what I do best!)
(video of water discharging from 10m2 flat roof onto double-pantile tiles)
If you look at 0:04, 4th tile from top (inc half tile at top) and
0:08, 1/3 up the video from bottom you will see a nice little bow wave of water lapping up to the tile joint to the left.Now, it would have to get past 2 more small ridges to leak, but today's rain whilst moderately hard was not torrential.
The other half of the dormer roof has the same arrangement but the water hits the other half of the tile profile so any bow wave would wash clean over the joint top rather than into it.
No evidence of leaks inside, or damp membrane.
Just wondering if I should maybe put an corner onto the end of that gutter to send the water stream left by half a tile. Or not bother...
How tolerant are interlocking tiles of this sort of abuse?