Inbound traffic keeps arriving on the backup ISP despite AS path prepending

R1 (AS 64500) announces 203.0.113.0/24 to ISP1 (AS 64496, primary) and ISP2 (AS 64497, backup). ISP1 and ISP2 peer with each other. To keep inbound traffic on ISP1, the team prepends 64500 64500 64500 toward ISP2. Outbound already prefers ISP1 through local preference. Even so, about a third of inbound traffic arrives on the ISP2 link, and the backup circuit is billed on usage. Management asks why the prepend "does not work".