BSC – Fixing ‘TransferHelper: TRANSFER_FROM_FAILED’ Error Even After Approval

I'm rying to swap some BNB -> Token, then swap it back, all in one solidity function. Here's my code:

interface IERC20 {
    function approve(address spender, uint256 value) external returns (bool);
}

interface IPancakeSwapRouter {
    function swapExactETHForTokens(
        uint256 amountOutMin,
        address[] calldata path,
        address to,
        uint256 deadline
    ) external payable returns (uint256[] memory amounts);

    function swapExactTokensForETH(
        uint256 amountIn, 
        uint256 amountOutMin, 
        address[] calldata path, 
        address to, 
        uint256 deadline
    ) external returns (uint[] memory amounts);

    function WETH() external pure returns (address);
    function factory() external pure returns (address);
}

interface IPancakeFactory {
    function getPair(address tokenA, address tokenB) external view returns (address pair);
}

contract Test {

    receive() external payable { }
    fallback() external payable { }

    function BuySell(address token_out, uint value) public payable {

        address swap_router_addr = address(0x10ED43C718714eb63d5aA57B78B54704E256024E); //Pancake router
        IPancakeSwapRouter swap = IPancakeSwapRouter(swap_router_addr);

        uint deadline = block.timestamp + 1200;

        address[] memory path_buy = new address[](2);
        path_buy[0] = swap.WETH();
        path_buy[1] = token_out;

        // 1. try buy 0.001
        uint256[] memory amounts;

        amounts = swap.swapExactETHForTokens { value: msg.value } (
            0, path_buy, address(this), deadline
        );

        // 2. try to sell 
        // approve first
        IERC20 token_dest = IERC20(token_out);
        address pair = IPancakeFactory(swap.factory()).getPair(swap.WETH(), token_out);
        token_dest.approve(pair, amounts[1]); // approve WBNB/Token pair
        token_dest.approve(swap_router_addr, amounts[1]); // approve PancakeSwap Router

        // sell
        address[] memory path_sell = new address[](2);
        path_sell[0] = path_buy[1];
        path_sell[1] = path_buy[0];

        swap.swapExactTokensForETH(
            amounts[1], 0, path_sell, address(this), deadline);
    }
}

I can confirm the error occurs at the last line swapExactTokensForETH, it says "TRANSFER_FROM_FAILED", I guess it's about approve(), but I already approved the token for PancakeRoute before the swap.

Some failed tx sample: https://bscscan.com/tx/0x3feaa9c746ee4458d073a0311b90fb8658d45ed7bac4877bc2934f7e05c4c538

Why the error?