Why does my iPad require me to tap 'OK' for a 6 digit PIN, but my iPhone does not?
On your iPhone, you have a 6 digit code, replacing the prior 4 digit code.

On your iPad, you have an alphanumeric password, which just happens to consist of just 6 digits. Alphanumeric passwords on iOS are just like other passwords in that you have to confirm when you have finished typing.