Have you checked the "work" tables ? Even "saved/work" documents throw a record in the RM20201 | Open Transaction Apply.
- RM10101 – Distribution Work and Open
- RM10201 – Work/Unposted Cash Receipts
What about the original invoice to which this cash receipt is applied, have you checked that the current transaction amount of the invoice has been deducted with the cash receipt amount ?
If so, please go for check links. As long as that doesn't get you issue solved out, and you are sure that it is corrupted and can't be reached anywhere on the screen, database correction may be required.
Although, I may as a question in here. You indicated, "another" weird issue, how often do you encounter such issue in your Dynamics GP environment, in case this is a frequent problem, I would consider troubleshooting the root cause of the problem to stop the bleeding.
Your feedback is highly appreciated,
