Unable to get bot events for multi-party DMs (MPDM/MPIM)

[avrin-123]

I have a slack app bot installed in my slack workspace. My Slack bot isn't responding to @mentions in multi-party direct messages. Even though I've:

1. Added the required bot scopes - mpim:history, mpim:read, mpim:write
2. Added "message.mpim" in the bot event under event_subscriptions

The bot events aren't being triggered when the bot is @mentioned in these exisiting group conversations. What could be causing this issue? I am also not able to invite the bot in to this existing group chat.

Steps to reproduce:

1. Existing multi party direct message with User1, User2 and User3
2. @ mention the bot app in the conversation

Expected result:

Receive event of type message.mpim. So that the event can be handled and respond back.

Actual result:

Not receiving message.mpim event