It looks like your input file is all one "line", and there's a hard limit in awk for the number of fields (space separated since that's the default behavior without '-F' being used with awk).
I imagine that sed, being stream oriented, won't have this issue, so I'd try
In the meanwhile I'd transformed my flat file in a text file (with cr+lf), where one record is 1500 chars long.
In this instance I'd like to use the awk command, but I obtain the same results (note that your suggestion works even in this case...). It's only to understand where I made an error.