Solidity – How to Convert Hash Function Hex to Bytes32

The output of hash function SHA256 is a hex string of size 64. For example: 3ed54831f488a22b28398de0c567a3b064b937f54f81739ae9bd545967f3abab

I want to call the function of a smart contract with the following structure:

function register(bytes32 hash)

and pass the hex as an argument.

I use the web3 JavaScript library.

First I tried to pass the argument without any conversion. The result was a truncation of the last 32 bytes

Then I tried web3's fromAscii function with the same results.

I also tried hexToBytes and bytesToHex functions from the 1.x version. hexToBytes returns a byte array of size 32. When I pass the array to the smart contract function the results are totally different from the expected ones.

So how do I convert a hex of 32 bytes (64 character hex) in a format suitable for a bytes32 function argument ?

I know I can use bytes or string instead of bytes32 but I would rather avoid it as the hex of SHA256 is indeed 32 bytes.

EDIT

I create a test contract and test it in remix. I call set function with input 0x3fd54831f488a22b28398de0c567a3b064b937f54f81739ae9bd545967f3abab. The value is not truncated and I get the expected results.

pragma solidity ^0.4.24;

contract Test {
    bytes32 public hash;

    function set(bytes32 h) public {
        hash = h;
    }
}

So, the problem should be in the web3 library or JS itself right ?

UPDATE

I use the truffle-contract library to call my smart contract's functions