"revert erc20 transfer amount exceeds allowance" error with transferFrom() but not with _transfer()

Question

I confirmed that approval is accurate, yet I receive the "revert erc20 transfer amount exceeds allowance" error when using the transferFrom() function. Yet for some reason the internal _transfer() function works without any issues.

I checked and transferFrom() is public, not external.

The following code results in an error:

    function addLiquidity(uint256 _tokenAmount) external payable onlyOwner {
        // Transfer the tokens from the sender to the contract
        // Sender should approve the contract spending the tokens
        require(allowance(msg.sender, address(this)) >= _tokenAmount);
        transferFrom(msg.sender, address(this), _tokenAmount);
    }

Where this works:

    function addLiquidity(uint256 _tokenAmount) external payable onlyOwner {
        // Transfer the tokens from the sender to the contract
        // Sender should approve the contract spending the tokens
        require(allowance(msg.sender, address(this)) >= _tokenAmount);
        _transfer(msg.sender, address(this), _tokenAmount);
    }

Note the require statement before each attempt to transfer that passes and therefore clearly shows the contract is allowed to transfer the tokens.

Why does _transfer work but transferFrom does not? Shouldn't transferFrom work, especially considering the allowance has been correctly set?

Answer 1

I really don't think that the allowance are trully set. Because _transer doesn't require the allowanceanywhere. Can you share the _transferFrom?