Yes this is correct. So to confirm the proration is not working on the actual payment/money spent, it's working on the price from the plan itself.
In order to achieve accuracy, shouldn't it be calculated from the users payment and not the plan amount since that can always be changed? You mentioned a bug ticket and I appreciate that, will it resolve it in such a way that this wouldn't happen again?

Thank you kindly!

Let me give you a scenario in which this makes no sense:

Company offers a discount on a plan of $100 per month plan (company is not giving anything, they're simply "allowing" a lesser cost).
Discount = 50% OFF 1st Month
User pays $50 for first month
Later that day user decides to upgrade to better plan of $200 per month (no discount is offered on such plan)
User now receives approx $100 (maybe $99.change) towards a plan with no discount available

In this case as you can see the user would receive the plan which should be $200 for only $150. This is faulty logic because the user should only get a discount if it is offered.

I truly appreciate you all discussing this internally. I honestly can't think of any time where leaving the proration on the plan itself instead of the user's payment is a good thing. The only way to properly make sure the user always gets what they paid for && the Company is protected against such losses, would be to prorate their payment instead of the plan.
I understand why you'd want to give us the option to select which way we prefer, however also I believe it is redundant. Meaning if we're using the user's payment and the plan is $100, the proration will still be on the correct value which is in this case $100. In other words, the payment option would always be correct whereas the current method will only be correct sometimes.
Thank you all so much for implementing this at least as an option for us to decide. I do hope this helps to see that may not be necessary and may require more code from you all, but you guys have your reasons & a great product, so thank you for whatever you decide.