The following test fails on the last line:

    #[test]
    fn test_current_pos() {
        let offset: u64 = 256u64 << 30;

        let mut cipher = chacha20::ChaCha20::new(&Default::default(), &Default::default());
        cipher
            .try_seek(offset - 64)
            .expect("Couldn't seek to nearly 256GB");
        assert_eq!(cipher.try_current_pos::<u64>().unwrap(),
            offset - ((1<<6))
        );
        cipher
            .try_seek(offset - 63)
            .expect("Couldn't seek to nearly 256GB");
        cipher.try_current_pos::<u64>().unwrap();
    }

The root cause appears to be a combination of the incorrect looping counter behavior highlighted in RustCrypto/stream-ciphers#391 and from the behavior of try_seek in StreamCipherCoreWrapper. try_seek sets the block position to (((256u64 << 30)-63) / 64) as u32, which is u32::MAX, and then calls write_keystream_block, which increments the counter, causing it to loop back to 0. Then the try_current_pos calls get_block_pos(), which returns 0 -- see

SeekNum::get_block_byte is then called with a value of 0 for the block parameter, and the checked subtraction on this line returns None.