Without being funny did you really see MAN FLX or was that what you expected to see (we've all done it)? It is just that I cannot find a ref to that. If however you were looking for MAN FLX on TO and instead got MAN THR with an amber box the reason is in DSC-22_30-100 (in my book) and it says
MAN THR
White with amber box
"A/THR is armed, and the most advanced thrust lever is above CL detent (two engs operative), or one above MCT/FLX (engine out) and not in a detent.
Only one of three instances of amber boxes in column 1 I can find. The other two being mentioned above.