Gawk gsub puzzle

[Celsus English]

I'm using GNU Awk 3.1.3.

Am I missing something here in this very simple test ? .....

Infile.txt contains
AAABBB
CCCDDD
AAACCC
BBBDDD

TestGAWK1.bat contains
gawk -f Changes.txt Infile.txt > Outfile.txt

Changes.txt contains
{gsub(/AAA/,"ZZZ");print}

When I run TestGAWK1.bat, Outfile.txt as expected contains
ZZZBBB
CCCDDD
ZZZCCC
BBBDDD


Now, the plot deepens ...

TestGAWK2.bat contains
gawk {gsub(/AAA/,"ZZZ");print} Infile.txt > Outfile.txt

When I run TestGAWK2.bat, Outfile.txt contains
BBB
CCCDDD
CCC
BBBDDD

The substitution didn't work ! Any help appreciated.

Regards,
Celsus English

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