This looks like sunburn to me. That is my opinion. The sun changes. What you thought was 50% shade might have changed with the season.
The only other explanation is possibly a contaminated compressed sprayer? or as Yew Sung noted ...chemical drift from a neighbor.
I can show you pictures of the exact same nature when I moved a vanda that was hanging in shade and pulled it more up front. It suffered the same looking damage on one entire side. It is ok...but looks terrible.