Problem with subdivisions in sculpting

At the beginning I want to say that I am a totaly neewbie and I’m learning to work in Blender from the tutorials. I have a problem with my model in sculpting mode.
This is how it looks like on 8th subdivision level.
And this is how it looks like on the 7th subdivision level. From the couriosity I clicked on the 8th level and saved project. Now I figured out that I can’t go back without this issue. Before on the 7th level it looked fine. If there is any possibility to go back to the 7th level? This 1 level difference makes 2mln polygons so I will be grateful for any help because my pc can’t make it with so many polygons. I want to make my project in low poly. After that it will be reptology stage. During reptology this issue will be annoying? If no, maybe I should hide the hair layer and continue and with low poly it will disapear?

EDIT: And one more question about sculpting. What looks like the best (and easiest?) workflow? Is the multiresolution and subdivisions are better than dyntopo or this second option is better? Mine tutorial is about this first option but it is quite old.

This looks like the infamous spike bug of the multires modifier.

It’s not happening often (fortunately), but when it happens the problem is that you can’t get rid of this , as fixing it by example with smoothing the spike in one multires level will see it re-appear in a lower/higher multires then, then back re-appear in the multires level you should smoothed it out.

It’s a very bad bug, but as no developers is working on multires, it’s been there since a lot of time and will probably stay here for a while

Only solution is to save regularly into different file if you need to work with multires, so if the spike bug happens, you can load a previously saved blend.

Now if you’re getting it and do not have any way to get a previous state of your sculpt, retopo + trying to rebuild the multires levels with shrinkwrap is the only solution.

Personnally i prefer Dyntopo over multires, because it feels much more natural to me to sculpt with Dyntopo.
Multires has the advantage of allowing to give the sculpt an armature so you can pose, something not possible with Dyntopo (unless you try to work around with mask brush and twist to rotate/grab to move/etc…, not really practical), and multires allows to paint on the sculpt if you unwrap its base (something that can’t be done with Dyntopo as it’s not keeping data like UV etc…)

But in term of sculpting i fnd Dyntopo simply much more convenient and usable and more enjoyable to work with (and no spike bug ruining your sculpt).

After that it’s going through retopo+bake anyways if i need something usable out of the sculpt mode.