You may need to open an SR for this.
Another thing to check for is the other modules -- if you receive to 6 or 8 decimals, can the Payables invoice reflect that number of decimals? If not, there could be issues such as being unable to close the PO because the last decimals can't be invoiced. E.g. 8,66666633 were received but if AP shows 8,6666663 then the PO will show the last 00000003 as unbilled, and prevent closure. There is an enhancement request under consideration to keep the number of decimals in sync between modules, but to my knowledge it hasn't been implemented yet, so I wanted to make sure you knew to test for that once the general issue of getting to 8 decimals is resolved.