Random 1-frame offsets with `syncTargetLine` and `syncSourceLine`

[LightArrowsEXE]

I'm syncing an OP and an ED to an episode using subkt. However, using syncTargetLine and syncSourceLine, some lines appear to be randomly offset by 1 frame (both positive and negative). This issue persists even when doing a 0-frame shift, and happens on both regular lines and typesetting. The offsets are easily discernible in both Aegisub (Daydream Cafe 9213, release build) and mpv (0.33.0-217-gf049acfd43, shinchiro's build).

Below is our current subkt setup and relevant subtitle files. Video and other missing files can be found you-know-where, and you can ask @rcombs for repo access if that helps. Jahy.zip