I have posted this as a comment at Volker's post * on IBM Technote 1267071: Changes in Lotus Spreadsheets document do not save
Regarding the issue Volker raises here, I believe I have found the problem and there is a VERY simple workaround and this issue is probably unlikely to be seen in most real world uses.
If you create a new spreadsheet or open an existing one, go to a cell and type something. If you click into another cell or hit tab or enter and then click the "x" to close the spreadsheet, then you will always receive the prompt to save the spreadsheet. If you just type something in a single cell but don't navigate to another cell, then you will not receive the prompt to save the modification. A bug indeed, but the IBM Technote should be updated. I will also be notifying Lotus via a response to the technote with how to reproduce this and also to hopefully modify the technote with a note to tab out of the field to receive the prompt for saving the modifications.
* It's ironic that I'm responding to someone who is "neither a business partner, a design partner nor a customer" of the Lotus products, but this is how Volker describes his involvement with Lotus products. I felt that since so many people will see the issue he raised that I would also let you know how to easily avoid the issue that was pointed out by someone who claims to not be a user of the product.