RESOLVED:
I just stumbled across this after half a day - racking my brains...
1.6.11 sprintf() chews up (at least one of) the incoming values.
Even tho the comments suggest it's the avr-gcc tools...
I'll probably go back to 1.6.7 for now, as it worked fine - just trying to stay up to date. - sigh.
SOLUTION:
The bug arrived with Windows 10 Anniversary update (surprise!)
I restored back prior to the update, thence reinstalled 1.6.11
Everything working again as it is expected to!