Hi whereas-stuff,
Thank you for the file and information.
Hair collisions in C4D only happen while dynamics are running — the program steps through time in little slices (sub-steps) to check if hairs hit objects.
When you use the Brush while the scene is paused, you’re instantly moving hair from point A to point B with zero time passing — so the collision system never gets a chance to check.
Even during playback, if you move hair very fast, it can skip past an object between collision checks unless you increase sub-steps in the Hair or Project Dynamics settings.
For grooming, the Brush is meant for art direction, not physics — if you need collision during grooming, play the simulation slowly and brush gently.
Consider (later) enableHair> Dynamics> 'Rigid' mode to lock the style once it’s where you want.
In short, movement in no time is like infinite speed, like mph, km/h, or 1/0. In all cases, Simulations are not matching reality; they aim to find the best compromise between reality and computer power, typically with iterations or sub-steps, like 20 per frame.
All the the best