Hello,

Please first upgrade your plugin. Then try the following code.

It will override placeholder image for user by member type default avatar.

/**
 * Override avatar moderator placeholder url by member type associated avatar
 *
 * @param int    $user_id User id.
 * @param string $type    Avatar type.
 *
 * @return mixed
 */
function buddydev_get_member_type_associated_url( $user_id, $type = 'full' ) {
	$active_types           = bpmtp_get_active_member_type_entries();
	$associated_avatar_urls = wp_list_pluck( $active_types, 'associated_avatar_urls', 'member_type' );

	$user_member_types = bp_get_member_type( $user_id, false );

	// Empty member return default url.
	if ( empty( $user_member_types ) ) {
		return '';
	}

	$url = '';
	foreach ( $user_member_types as $member_type ) {

		if ( empty( $associated_avatar_urls[ $member_type ] ) || empty( $associated_avatar_urls[ $member_type ][ $type ] ) ) {
			continue;
		}

		$url = $associated_avatar_urls[ $member_type ][ $type ];
		break;
	}

	return $url;
}

add_filter( 'bpavmod_placeholder_avatar_url', function ( $placeholder_url, $item_type ) {

	if ( ! function_exists( 'bpmtp_member_types_pro' ) || 'user' !== $item_type ) {
		return $placeholder_url;
	}

	$user_id = bp_is_user() ? bp_displayed_user_id() : bp_get_member_user_id();

	if ( ! $user_id || ! bpavmod_is_in_moderation( $user_id, 'user' ) ) {
		return $placeholder_url;
	}

	$url = buddydev_get_member_type_associated_url( $user_id, 'full' );

	return $url ? $url : $placeholder_url;
}, 10, 2 );

add_filter( 'get_avatar_url', function ( $avatar_url, $id_or_email ) {

	if ( ! function_exists( 'bpmtp_member_types_pro' ) || ! function_exists( 'bpavmod' ) ) {
		return $avatar_url;
	}

	$user = is_email( $id_or_email ) ? get_user_by( 'email', $id_or_email ) : get_user_by( 'id', $id_or_email );

	if ( ! $user || ! bpavmod_is_in_moderation( $user->ID, 'user' ) ) {
		return $avatar_url;
	}

	$url = buddydev_get_member_type_associated_url( $user->ID, 'thumb' );

	return $url ? $url : $avatar_url;
}, 15, 2 );

Regards
Ravi