Slicing (and other actions, but especially slicing) creates weird brown segments in areas that don’t make sense.
Like, I chopped across a model and got a bunch of brown/gold polygons in previously created holes.
I also tried to enlarge one hole via slicing and it completely filled it in with brown polygons…
Example
Those brown areas are the inside of the mesh. Think of it a bit like a sock thats pulled inside out when you slide it off your foot. In a nutshell, that’s what happens. Go into display settings, turning off ‘Two Sided’ will help ease this problem in the future but that artefact may remain there in some form because the outside is twisted up with it. The wireframe shows the mesh is very unbalanced, extreme amount of poly’s concentrated in one area and very few in another; this imbalance will cause issues like that. Try voxel remeshing this at a low count 300-400 to sort it out a bit and at least even the polys. After a voxel remesh, any bizarre aberrations that remain you can just clear away with the Trim tool.
Trim can fail sometimes, it can depend on the geometry/topology of the mesh.
It happens in more even meshes too. I’ve just recently been doing more work on tweaking mini sculpts by other artists instead of working on my own. Remeshing would destroy the sculpt.
I can’t see how summoning broken topology where there was nothing isn’t a bug, even if the surrounding area is uneven.
I’ve never had inverted mesh once throughout any of my properly developed sculpts (happens a lot with play) because I keep the mesh as even as I can and i keep polygons comfortably confined to their boundaries without overstressing the mesh grid - another way that can happen and will happen with an even mesh because it creates an imbalance in the mesh grid, and well, makes it uneven afterward. But everyone’s different with their approach to modelling. Remeshing will ruin it, yes, because its uneven - again, a factor I don’t have a problem with because I can comfortably swap back and forth from lower resolutions back to high without it being detrimental to the model. If the surrounding area was even, the trim tool would have cut through generally without problems - again, something I do frequently. Since I don’t know exactly how and what you were doing at the time, it’s more complex to diagnose from just the picture. Just remember the polygon grid you see on the surface is just the surface, they actually mathematically connect all the way through the model and to the other side, there’s hundreds of thousands that you don’t see but are reacting when the mesh is manipulated with tools. You’ll also find an uneven mesh won’t display textures very well either if you get round to it and shadows go from sharp to fuzz as it swaps between higher to lower resolution across the mesh grid. Obviously an entirely 100% perfectly even mesh isnt possible especially with dynamic topology but I use that to only enhance surface detail without subdivision, and even then i have to be careful as it starts to stretch and pull out surface polygons to their boundaries quite quickly. But that’s when I’m working with 13-14+ million vertices in Nomad, which is a threshold my models get to from complexity in detail with me even being sparing. It’s an area I understand well hence why the advice. Hope you managed to absolve it either way, and good luck with your work 
