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.

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 2andDisc 3will be added to the playlist, butDisc 1will 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.