Record truncation? Not normal behavior unless the record has embedded ascii nul characters. Lack of disk space or exceeding enabled quotas will also cause the output file to truncate.

grep has a line length limit of 2048 characters.
There also is a concept of largefiles, files which are so big a signed 32 bit file pointer cannot access them > 2.4GB.

Which of these things applies to your case?

The way disk i/o in unix works is that data is parked in an in-memory cache in the kernel - it is not guaranteed to be written to disk when the write() system call is invoked. Every 30 seconds or so the syncer daemon issues a sync command. This forces the kernel to write everything in the kernel buffer to disk.

What you are seeing is an incompleted write operation - for whatever reason. Common reasons are - a signal was sent to the process that terminated it, write() or sync failed because something else filled up the
disk (maybe a temp file) and then that file went away, disk errors caused a fatal error. If it's an nsf mounted disk then the network also becomes an issue. What errors do you see in the log?