At least on one proprietary subtitling program I use, the default minimum gap between subtitles is two "clear" frames, a frame gap of two.
The reason has not to do with SMPTE, or ISDCF, but with subtitling and the fact that less than that may trick the eye/mind and miss the change of subtitles, especially when the form is similar.

The guideline is similar to the one that says that subtitles should change on a change of scene, even though the later is less disorienting when the subtitles don't start shortly before or end shortly after the change of scene.

There are different schools, where that number (2) may be up to four (4), but I have never seen a smaller number, as you describe on your second example.