Is it possible to create a file given an md5 hash

You can use dd to pipe only the first 512 bytes to md5sum. However this will cause md5sum to be oblivious of the filename, so in addition replace - with the filename again.

find . -type f -exec sh -c "dd if={} bs=512 count=1 2>/dev/null | md5sum | sed s\|-\|{}\|" \; | sort -k 34;

There's a wrong assumption here:

As far as I know rsync automatically verifies the transfer went well with hash checks after the transfer is completed.

Rsync uses checksums to determine if a sync is needed. But, Rsync does not reread the created copy, it trust the kernel to report errors. So, the conclusion is simple: the files are not identical. Could be just one bit, could be more. How much mismatch there is, a checksum doesn't tell you.