TVOut+Interlacing shader. It’s not occur with NTSC ones. Occur both with Beetle and Duckstation. I use a custom resolution (1920x480p) 60hz and a PC CRT. Integer scaling is On.
Beetle appears to report incorrect resolutions and be plain broken with PAL, can’t say anything about Duckstation atm.
You could try PCSX-Rearmed, PAL games on the PS1 would often make use of the increased resolution though, which requires at least 512p with your method.