Why does awk erase part of a file?

Question

linux

I want to change the text according to the regular expression in the files
I write the following:

ag -rl regexp | xargs awk "{gsub(/regexp/, \"string\"); print > FILENAME}"

usually works fine, but on large files it cuts off the end of the file.
Why can this happen and how to fix it?

Answer 1

Alas, I could not figure out how to fix it, but I found an alternative (sed came to the rescue)
the -i switch indicates in-place replacement and overwrites the file.