The Store Tries To Add The Products To The Cart After Placing Order, Cart Session Not Cleared, Help!

Hello everyone,
I've noticed a strange behavior in our store that I cannot seem to be able to solve. 4.9.2.SP4
This is how it usually happens:
1. User adds something to the cart (logged or unlogged)
2. Goes to cart/checkout or directly to checkout.
3. Tries some shipping/payment combinations, there are quite a few dependencies between them and they are tangled, I will describe them below.
4. Finishes the order, usually using myPOS, Stripe or Paypal, but I'm pretty sure it happens with CoD and Pick-up payments.
5. In case they choose online payments, they are transferred to PayPal or myPOS respective pages.
6. Or they finish the order.
7. They get to the order landing page, and here comes the problem:
8. The website tries to add the products that were in the cart BEFORE the checkout, some of them are obviously out of stock since the order decreased their number in the inventory, and the user gets a message some products are missing.
9. At the same time, if I go to the cart pages, I can see the user cart along with his order number, and they are the same (as long as the products had more inventory.

As you may guess, this leads to some frustration on the customer side, but it also leads to some other problems, since the next time they log in their account and add something to the cart, the products that are still available from previous order are still in the cart, and some people fail to notice and place orders with products they've already got from us.

I'm not a programmer, I have some basic knowledge of the CS CART structure and I can follow some functions, but I never managed to fully grasp how exactly it happens.I've tried switching off some of the related add-ons, no luck, so it's either combination of them or it's not there. Seems that the cart session is not cleared.

So back on the dependencies I mentioned:
1. Payment methods have loyalty group dependencies (Simtech Development: Loyalty program add-on)
There is a Stripe add-on (since our CS Cart is older version), pretty old one
There is a myPOS add-on for myPOS payments, also old one and the reason I'm not upgrading sinc e I doubt it will work and we can't stop taking myPOS payments.
We have set some offline payments (like CoD and Pick-up) and PayPal.

2. Shipping methods have some dependencies (using CS CART Payment dependencies add-on) basically to prevent people form other countries using CoD, for example)

3. Some categories have some payment method restrictions (Webkul Payment Method Restriction), but I don't think this is the problem, it happens with just about any product.

Site is in Bulgarian and I only have a live site right now.

Any help to locate the issue and find out why the cart session is not cleared and people get their carts back AFTER checkout is welcome.

Hello

After finish the order, is the order status in the admin panel Processed? (P)

Best regards

Robert

Hello

After finish the order, is the order status in the admin panel Processed? (P)

Best regards

Robert

I barely remember the old statuses.
There are two statuses according to whether it was paid online or it uses offline payment:

- Paid (which would be the old P/Processed)
or
- Accepted if it uses offline method for payment (which should be the basic status, O/Open?)