All bullets bulge a case a little. If the bulge gets too wide, the Lee Carbide Factor Crimp Die (CFCD) will rub the wide spot, marking it as the case passes through the carbide sizing ring, which is trying to ensure no cartridge is too wide to feed.
Cases designed for self-loading pistols typically have a tapered thickening of the brass below where bullet bases are supposed to stop in them. This thickening prevents an unsupported portion of the case, as some chambers leave exposed, from blowing out on firing.
If you seat a bullet past the point where the base is supposed to stop, it enters that thickened portion, bulging it out farther than the thinner brass does, and this forms an extra-wide bulge at the bullet base. Since that happens during seating, it occurs inside the CFCD, and then that extra thick part is too fat to slip through the carbide sizing ring when you withdraw the case. Thus, the ring sizes it down, making the rub mark you see as that ring.
The bottom line is the ring is caused when using the Lee CFCD, and the bullets are seated deeper than the case designer intended. Stop doing that, and the ring will no longer be made.
Does this weaken the brass? Yes, a bit. Usually, it is not critical as most of the taper is not in the unsupported part of a chamber, but the case is a little weaker at the bullet base, and I would avoid doing it since the cure is so simple. Again, just stop seating too deeply.