this is not properly said a bug, but a limitation inherent to what a IDX/SUB is.
IDX/SUB are 4 colors bitmap images encoded in RLE, with a given output resolution
So if a IDX/SUB is done for PAL, its bitmaps have the resolution of PAL (720*576). If using this SRT for a NTSC segment (720*480), the vertical resolution of the subtitle will be higher than the resolution of the underlying video - the best case is that you will not see the bottom of the subtitles, the worst will be the freeze of the player.
The SRT files doesn't have such problem, as it is text we render at the required resolution.
To use any IDX for any output standard, we need to build up a decent subtitle images resizer, which is not as simple as it look. This will be done one day, as soon as our schedule will allow us to do so.