Alan
It seems very likely that R11-004 is the one miscoding 50FF5F. I received it again today, now climbing from FL89 into FL359, yesterday it was descending from FL380 into FL78. It matches the pattern for my reception on ESKN airport.
R11-004, Ilyushin IL-78MP, c/n 0063467027, Pakistan Air Force (delivered in april 2012)
Niklas