Whoops, I just realized it’s this plugin that sends out the emails:
https://wordpress.org/plugins/buddypress-group-email-subscription/

Hey Brajesh, awesome. Very kind of you.

This looks like the activity filter, but I don’t see anything as good for forum posts. I’ll check with the developer and see what they say.

/**
		 * Filters whether a given user should receive immediate notification of the current activity.
		 *
		 * @since 3.6.0
		 * @since 3.8.0 Added <code>$subscription_type</code> parameter.
		 *
		 * @param bool   $send_immediately True to send an immediate email notification, false otherwise.
		 * @param object $activity          Activity object.
		 * @param int    $user_id           ID of the user.
		 * @param string $subscription_type Group subscription status for the current user.
		 */
		$send_immediately = apply_filters( 'bp_ass_send_activity_notification_for_user', $send_immediately, $activity, $user_id, $subscription_type );

		/**
		 * Filters whether to add the current activity item to the digest queue for the current user.
		 *
		 * @since 3.8.0
		 *
		 * @param bool   $add_to_digest_queue True to send an immediate email notification, false otherwise.
		 * @param object $activity_obj        Activity object.
		 * @param int    $user_id             ID of the user.
		 * @param string $subscription_type   Group subscription status for the current user.
		 */
		$add_to_digest_queue = apply_filters( 'bp_ges_add_to_digest_queue_for_user', $add_to_digest_queue, $activity, $user_id, $subscription_type );

Just posted an issue here: https://github.com/boonebgorges/buddypress-group-email-subscription/issues/191

Hi Brajesh, boonebgorges gave a nice response here and invited you to chat with him directly if you like:
https://github.com/boonebgorges/buddypress-group-email-subscription/issues/191#issuecomment-606077130

Over the last few days I’ve been finding that the “All email” subscription option has been sending correctly, e.g. when a reply is approved (restored). Not sure if that’s true for group activity, though, or if it applies to digest emails as well. I’m happy to help test and run down the specific situations where there is trouble.