I certainly wouldn't rule out temperature as a cause and I don't have numbers but I still suspect the difference in solar heating between composite and aluminum will not be vast and given the number of these in service (over 6,000?) if there was a marginal temperature effect we would have seen it by now in something else.
I'm guessing of course but suspect it will turn to be either really bad luck, some installation issue or - and my favorite - that the version of this ELT used in the 787 has been 'tweaked' in some way that has left it vulnerable.