Found the (likely) reason for a weird subtitle desync. Video transmission is weird.

Both 23.976 and 24.0 fps videos use the same type of time code (SMPTE 24). When using 24.0 fps video and SMPTE 24 time code, it syncs with a real time clock (similar to drop frame time code). When using 23.976 fps video with SMPTE 24 time code, it does not sync with a real time clock (similar to non-drop frame time code).source

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