Renewing subscriptions with next payment date in past (ex missed payment) can cause new profile start date in past

[dparker1005]

Describe the bug
This is the function where we set the profile start date to (potentially) a date in the past:

pmpro-proration/includes/checkout.php

Lines 286 to 330 in e2834c4

	/**
	* Set the date that the first recurring payment will be charged.
	*
	* @param string $startdate The start date for the membership.
	* @param object $order The order that is being purchased
	*/
	function pmprorate_set_startdate_to_next_payment_date( $startdate, $order ) {
	global $current_user;
	// If using PMPro v2.x, we don't have the PMPro_Subscription class. Use old logic.
	if ( ! class_exists( 'PMPro_Subscription' ) ) {
	//use APIs to be more specific
	if( $order->gateway == 'stripe' ) {
	remove_filter('pmpro_next_payment', array('PMProGateway_stripe', 'pmpro_next_payment'), 10, 3);
	add_filter('pmpro_next_payment', array('PMProGateway_stripe', 'pmpro_next_payment'), 10, 3);
	} elseif( $order->gateway == 'paypalexpress' ) {
	remove_filter('pmpro_next_payment', array('PMProGateway_paypalexpress', 'pmpro_next_payment'), 10, 3);
	add_filter('pmpro_next_payment', array('PMProGateway_paypalexpress', 'pmpro_next_payment'), 10, 3);
	}
	// Note. This is not MMPU-compatible for PMPro v2.x.
	$next_payment_date = pmpro_next_payment( $current_user->ID );
	if( !empty( $next_payment_date ) )
	$startdate = date( "Y-m-d", $next_payment_date ) . "T0:0:0";
	return $startdate;
	}
	// Get the level ID that the user is switching from.
	$clevel_id = pmproprorate_get_level_id_being_switched_from( $current_user->ID, $order->membership_id );
	// Get the user's current subscription for that level.
	$current_subscriptions = PMPro_Subscription::get_subscriptions_for_user( get_current_user_id(), $clevel_id );
	// If the user doesn't have a subscription, bail. This shouldn't ever happen though since we check for this earlier.
	if ( empty( $current_subscriptions ) ) {
	return $startdate;
	}
	// Return the next payment date.
	$current_subscription = current( $current_subscriptions );
	return $current_subscription->get_next_payment_date( 'Y-m-d H:i:s' );
	}

Option 1: Set next payment date to today instead of to the past - The issue here is that we may as well just charge the billing amount at checkout

Option 2: If $next_payment_date is in the past when calculated in the pmprorate_pmpro_checkout_level() function, just bail from the function. This feels like it makes more sense. If a payment was missed, try to treat this checkout as a "new checkout".

Isolating the problem (mark completed items with an [x]):

• I have deactivated other plugins and confirmed this bug occurs when only Paid Memberships Pro plugin is active.
• This bug happens with a default WordPress theme active, or Memberlite.
• I can reproduce this bug consistently using the steps above.

WordPress Environment

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