Librorrent doesn't spot a corrupted file

I have a torrent file. I can modify one of the files, its ending, and the force recheck in libtorrent doesn't spot that it's invalid (it thinks it's valid). the block size I tried is 512kb and 256kb. Any ideas how to detect these situations or why it even happens? the diff is about 1kb at the end of the file (the file B adds 1kb of new content and the file A has just 26bits of the same content if I judge the hex diff correctly)