[GNC] Why did "Assign as payment…" remove splits and notes from my payment transaction?

[Jim DeLaHunt]

Hi, folks:

I have used little bits of the business features of GnuCash for a long 
time, but I don't know them well. I just tried to assign a Paypal 
transaction as a payment, and it looks like GnuCash changed the 
transaction in ways I don't understand.

I had an invoice for a customer. The invoice was posted, so my accounts 
receivable account had an entry for that invoice.

The customer paid me using Paypal. Paypal deducted a service fee. I 
entered a three-split transaction in GnuCash:

Debit $X    Asset:Paypal account, where $X is the gross amount of the 
invoice
Credit    $Y  Expense:Fees account, where $Y is the service fee Paypal 
charged
Credit   $(X-Y) Asset:Accounts Payable, where $X-Y is the net amount 
received

I selected this transaction, brought up the context menu, and selected 
"Assign as payment…". A dialogue appeared. It had the amount $(X-Y) 
filled in, so I changed it to $X. It had the date of the Paypal 
transaction.  The invoice was in the Documents list in the centre of the 
dialogue, so I selected that. I pressed the [OK] button.

Now, the transaction was changed. It had only two splits:

Debit $X    Asset:Paypal account, where $X is the gross amount of the 
invoice
Credit   $X Asset:Accounts Payable

My split with $Y for the service fee was nowhere to be found. Also, the 
Description for the transaction, and the Notes on each split, were 
changed to just have the customer name.

Q1. Is this the expected behaviour?

Q2. What does the "Assign as payment" of a payment transaction to an 
invoice actually do?  The GnuCash manual, section 7.6. "Process Invoice 
Payment", says what steps to take to perform an assignment, but it does 
not explain what GnuCash does in the course of the assignment.  It does 
not lead me to believe that it will rewrite a transaction, and discard 
splits.

Q3. What is the correct way to enter a transaction which pays an invoice 
and incurs service fees? Should I always create the transaction with 
"Process Payment…" from the business features if it will eventually be 
part of an invoice payment?  Is there a way to make an existing 
transaction an invoice payment without losing what I put into the 
transaction?

Q4. If I have an invoice marked PAID, how do I see which transactions 
are assigned as the payments for that transaction?

I am using GnuCash 5.8 on macOS Sonoma 14.7.2. The transaction and 
invoice in question were entered years ago, but I never bothered to 
assign the payment to the invoice until now.

Colour me confused.

       —Jim DeLaHunt