sounds like cat has it worked out , moisture condensation on the underside of the new valley.
new underlay would actually as a vcl to a point .
but not the be all and end all ..
There must have been an issue in the past for it to have been replaced .
you shouldn't be relying on the underlay to...
a little as above , the will be secret bedded. mortar out of sight .
this is good practice on a slate roof .
the other in the image posted is an abortion and the mortar on show will soon end up in the gutter.
Now, having said that there is at least one not laid correctly which will need a cut...
not all manufacturers make third round hips . these look like Marley major tiles . Marley for one don't do third round .
one comment , looks like hip trays are missing