I assume the hair isn't displaying when you add it again because the hair has 0 as it's radius. Meaning it is infinitely thin. How do you change the radius? I don't know.

As for the hair not being visible in the workbench, that's probably a bug. I can see the hair in the work bench.

@Alaska (Alaska) Hmm it's probably as you say, even if you don't remove existing hair, new is not visible. So will change tag to modeling instead. I don't see any hair radius setting for brush.

Yeah, there's no way to edit the radius yet. This stuff is all actively in development and not really ready for use yet.
So that part isn't really a bug. One important patch to follow is D14277.

I does seem there's a problem with the new object not being displayed in the viewport though, I'm not sure about that.

Yeah, it is the radius.

Quickie to change with python:

import bpy
C = bpy.context

radius = C.active_object.data.attributes['radius']
for dat in radius.data:
    dat.value = 0.05

• Workbench does not take hair shape into account (also shouldnt I think, also didnt do this in the old particle hair)
• Eevee did this in the old particle hair (but only if the shape was set to Strip and only using the "old" settings in particle hair Diameter Root and Diameter Tip)

So actually the issue is twofold:

• [1] The brush that adds curves adds them with zero radius
• [2] Eevee does not take the radius into account when displaying the hair as Strip

I actually think this should be two separate TODOs.
reg. [1] : @Jacques Lucke (JacquesLucke) , would it make sense to

• add an option Interpolate Radius to the brush? (or just silently do it when using Interpolate Shape?)
• when not using interpolation, maybe it would make sense to add them with something close to the old particle hair: this had 1cm at the root and 0cm at the tip

Regarding the two todos, I think that mostly makes sense, though I'd probably combine the setting for radius interpolation with the interpolation of all generic attributes.

EEVEE/workbench rendering of the new curves object basically needs to be rewritten, there is already a todo for that here: T96455