Take into account, that right after ECL startup,
compiler:*compiler-break-enable* is NIL:
So I suppose something sets it to T during compile-file.
I stand corrected. There is a function, FIX-READ-ONLY-VARIABLE-TYPE that contained a debug statement setting compiler-break-enable to T. Apart from that, I cannot see any other places where it might have been set. I will upload a fix.