Nothing wrong and not a bug.
When you elect to record a receipt or payment from with the Sales or Purchase Invoices tabs, by click the applicable button while viewing a specific invoice, Manager presumes you will probably satisfy the invoice in full. At the least, it knows the balance due on the invoice and can open the entry screen with that information prepopulated.
But when you start in the Cash Accounts tab to spend or receive money, the program doesn’t know where you’re headed. Even after you select Accounts receivable or Accounts payable, it doesn’t know, so it doesn’t fill in a customer or supplier. Add one of those; it still doesn’t know. You might be entering an advance payment, deposit, or funds transfer related to any invoice. Finally, you specify an invoice. Is it being paid in full? Who knows?
So rather than think of it the second situation as a shortcoming, think of the first as a little benefit. In either case, the amount is editable. One situation begins with knowledge of an invoice, so the program prepopulates the amount. You might need to change that. The other situation begins with knowledge of a cash account but no knowledge of the invoice, so it prefills nothing. At some point, the program could start guessing. But if it worked that way, there would be users flooding the forum complaining they were misled into entering incorrect amounts.