Have you or your neighbours seen any mushrooms near the plants? Just wondering about Armillaria, honey fungus. There's a picture and details here:
https://www.rhs.org.uk/advice/...
In some lists holly (Ilex) is not susceptible but the RHS have seen cases. Viburnum is susceptible so unless somehow they've avoided it so far it seems unlikely this would be the cause for the demise of the holly between the two healthy tinus, however.
Did you not have a very cold winter a couple of years ago there?