Slack Channel Integration with Amazon Lex Bot Not Working

After following all of the steps outlined in the LexV2 - Slack integration developer guide (https://docs.aws.amazon.com/lexv2/latest/dg/deploy-slack.html), Lex is not receiving any messages sent to our created bot within Slack. When testing Lex in the console, I receive the expected output. However, there is no response from the bot when a message is sent in Slack, and the Fulfillment Lambda function never even receives the event (even without a Fulfillment Lambda, no response from Lex via Slack). I have correctly set up the channel integration, providing the client id, client secret, and verification token from my created Slack app. The channel is configured on an alias which is tied to the latest version of my Lex bot.

My team has some existing bots that were configured in the same way months ago, and they are still functional. I copied all possible configuration from those Slack-Lex integrations to no avail. I have verified that the im_message event is being sent by Slack by providing an alternative event subscription request URL. However, the callback URLs from the Slack channel integration do not work. Any help would be appreciated, thanks.