Any update on this bug ?
The cut tool can make our lives so much easier, but it's not usable with the current state.

@Lance Phan (ssendam) wrote:

The hair is now completely messed up.

A few notes to somewhat resolve this broadly contoured observation.

Create a hair system on the default cube, set the emission number to 1.

Mine looks like this:

>>> solo = bpy.context.active_object.particle_systems['Solo']
>>> type(solo)
<class 'bpy.types.ParticleSystem'>
>>> for k in solo.particles[0].hair_keys : k.weight 
1.0
0.5
0.0
>>>

This is the visualization of the hair and its keys, using the weight paint tool while in Particle Edit mode. The root has a weight of one, the tip has a weight of zero and the midway key has a weight of one-half.

Go to particle edit mode, subdivide one or 2 segments of the hair.

I'll subdivide the segment between the midway key (weight 0.5: 'green') and the root key (weight 1.0: 'red')

>>> for k in solo.particles[0].hair_keys: k.weight
... 
1.0
0.0
0.5
0.0

>>>

This leads to non uniform hair dynamics - nice, if that is what is wanted, but, for users unaware of no-longer-evenly descending weights on the keys, it is, I suspect, the first step down the road to having completely messed up hair: Under gravity, the keys will fall at non-ascending rates. Short discussion: the hair will hang funny. This appears to be T67301 rearing its head.

Use the cut tool, click and drag from the tip of the hair.

Emphasis mine. Click (and no drag) evenly spaces the keys along the surviving length. Note that Blender preserves the key count: it does not remove keys during cuts. Click and drag, however, makes the interior keys - those between the tip and root - dance randomly.

The hair is now completely messed up.

Indeed.
Hopefully, these notes will be helpful to the occasional particle repairmen who happen by, now and again.
[Edit]
Neglected to upload the test file.