It's possible that the sun burn could be as a result of the decrease of melanin in the skin as advanced old age creeps up. There's also some correlation between melanin and the onset of Parkinsons disease but that's a little complicated and unlikely to have much relevance. Another cause for this strangely lop sided sun burn might be that perhaps you went to sleep for a significant portion of the flights with your elbow propped against the window slide and your head in the cupped palm of your hand. Of course, had you been awake and a line training captain, you could have toasted the other side on the cruise home.
Hope that helps.
Toodle pip.