The nicks I put into my strop while I was learning have been fixed, I'm still using it daily.

I put a small amount of contact glue under the flap and pressed it down resonably firmly (with a bottle or something similar) but not so hard that it dents the leather. I used one end of a jade roller that I use for my face because it's stays nice and cold (the other end for my face).

That made a nice tight glue down and no big lump where the glue is.

I then wiped off the excess from the strop and when it was dry I used a block or wrapped the bottle etc with fine w/d to make it smooth.

I did go over the whole strop gently so i didn't just have sanded spots.

I then wiped the strop with a damp cloth to get the sanding dust off it.

Then palm rubbed to get a bit of oil on it after it was dry.

I'm sure there are much better methods but this is mine and it works for me.

I had to fix that strop a few times, still do but very occasionally now.