Internal routers learn the ISP routes but nobody can reach the internet

AS 64500 has one internet edge router, R1, peering eBGP with ISP1 (AS 64496) on 192.0.2.0/30. R2 and R3 are core routers that run OSPF and iBGP with R1 on loopbacks. Last night an OSPF clean-up on R1 removed network 192.0.2.0 0.0.0.3 area 0, which the change ticket called "an old leftover". This morning every user behind R2 and R3 has lost internet access. R1 itself can still reach internet hosts. All BGP sessions are Established and R3 shows 2 prefixes received from R1.