Something is wrong with the current RDB database for Sony - Playstation (commit 7b37412, merged Aug 7). It may need reverting if they weren't a lot of changes.
In RetroArch, clear/remove the Sony PlayStation playlist, then scan the directory containing PSX ISOs. For multi-disc games, Disc 2 and Disc 3 will be added to the playlist, but Disc 1 will not.
I closed RetroArch, downloaded the previous version of the RDB (Commit 5d2b91a, April 23) and overwrote the RDB file in RetroArch > Database > RDB. After opening RetroArch, clearing and scanning the playlist again, all discs were picked up by the scanner and added to the playlist successfully.
