I might know why: the ticks become so near the graphic card glitch while drawing them.
Anyway, rules are better used to prevent bugs.
It only happed with Tick Rate 4 when BPM*SV*SVM < 20 and it happens because of some kind of internal variables, as it happened only coming down. It's something like a random glitch too, as redoing it made me obtain different results.
No need for a rule, neither banning SV-edited velocities as:
-No one, no one use 4 Slider Tick 4
-No one will want to use less than less of SV
-So there's no point in banning them and saying "isn't logical they're banned...?"
So, Hold Sliders are usable