Well, you have not explained what you did very completely. And what you have shown is confusing. Specifically:
- Your sales invoice does not appear to have offered an early payment discount. Unless you did, and the receipt qualified, no banner related to a credit note would appear.
- Your reason for deducting 1,700 from the original receipt is not clear. This was equivalent to giving the customer a 1,700 discount, so the original invoice amount was only 300.
- You have applied a 300 credit note, but the effect of that was to reduce the balance due to zero on the sales invoice. And the credit note was created at the end of May, 5 months before the sales invoice was raised. What was this for?
So, please, explain in detail what the circumstances of these transactions were.
If you had done things correctly, and the customer had paid within a 1 day early payment window, your sales invoice would have looked like this:

If the customer paid the 1,700 early, the modified sales invoice (which would not have been given to the customer) would then have temporarily looked like this:
The banner would have appeared on the Credit Notes tab list, looking like this:

When clicked, it would have expanded to this:
After confirming the new credit note, the sales invoice would have looked like this:

That version of the sales invoice could have been given to the customer, if desired.
To conclude, Manager is functioning as described in the Guides. But for some reason, you conflated early payment discounts with avoidance of automatic credit allocations through use of special accounts. When you wrote your first post, it seemed like you were simply raising two issues in the same post. It was not clear that you were trying to use the two features together. They have nothing to do with one another.
Special accounts can be used when customers make advance deposits. In such a situation, their purpose is to prevent advances from being applied automatically to intervening sales invoices. For example, suppose a customer made a security deposit equal to two months’ of rent. By default, Manager would apply that deposit to the next monthly rent invoice. But your goal would be to segregate the security deposit so the monthly rent invoices and receipts stood alone. That is what a special account can do.