AWS Lambda function invoked from another Lambda function runs stale request in queue

I have a Lambda function (A) that calls another Lambda function (B) and returns while the other one is still executing. Everything works perfectly locally (using sls offline), but when I deploy it, the following behavior occurs:

1. I call A for the first time. I see on Cloudwatch that A runs successfully and returns, but nothing on B, which apparently doesn't run.
2. I call A another time. Cloudwatch now shows that both A and B run, but the logs show that the invocation of B had request parameters corresponding to the first run of A.

Ensuing runs of A result in runs of B with request params from the previous run of A (e.g. B always lags behind by one and never runs the current invocation like it is supposed to, almost like each call of A pushes out the previous B invocation from a queue of size 1)

Some additional info:

• No VPC's are involved (if that makes a difference?)
• I am using sls

Anyone have any idea why this is happening? I know that AWS async invocations go into a queue, but I'm pretty sure they should just run ASAP if there's nothing in the queue.

Here's the code:

module.exports.A = async (event, context) => {
    context.callbackWaitsForEmptyEventLoop = false;

    try {
        ... some code ...

        await connectToDatabase();

        ... some code ...

        const newReq = req;
        newReq['newSessionId'] = savedSession._id.toString();
        const params = {
            FunctionName: config.apiName + '-' + config.apiVersion + '-B',
            InvocationType: 'Event',
            Payload: JSON.stringify(newReq)
        };

        lambda.invoke(params, (err, res) => {
            if (err) {
                console.log(err);
            } else {
                console.log(res);
            }
        });

        return {
            statusCode: 200,
            headers: {
                'Access-Control-Allow-Origin' : '*',
                'Access-Control-Allow-Credentials' : true 
            },
            body: JSON.stringify(savedSession)
        }
    } catch (err) {
        console.log(err);
        return {
            statusCode: err.statusCode || 500,
            headers: { 'Content-Type': 'text/plain' },
            body: 'Could not create session.'
        }
    }
}

module.exports.B = async (event, context) => {
    context.callbackWaitsForEmptyEventLoop = false;

    try {
        ... some code ....

        // Connect to Atlas
        await connectToDatabase();
        
        ... some code ...

        for (let i = 0; i < applicants.length; i++) {
            ... some code ...

            const savedResume = await DownloadResume(resumeId, accessToken, fileName, oauthSrcs.GSCRIPT);
            conversionPromises.push(ConvertAndExportToS3(fileName, savedResume, sessionKey));
        }

        await Promise.all(conversionPromises);

        ... some code ...

        return {
            statusCode: 200,
            headers: {
                'Access-Control-Allow-Origin' : '*',
                'Access-Control-Allow-Credentials' : true 
            },
            body: JSON.stringify('Converted all resumes successfully!')
        }
    } catch (err) {
        console.log(err);
        return {
            statusCode: err.statusCode || 500,
            headers: { 'Content-Type': 'text/plain' },
            body: 'Could not create session.'
        }
    }
}