At some point in time it started attacking every day. Soon after, it started attacking twice a day. It also breaks blocks by just walking into them. That, combined with it's speed, knockback resistance and no-fucks-given attitude even when it dips in lava, it's almost impossible to get away even on the surface, and completely impossible in caves or structures. It's far too resilient for it's "relentless pursuer" archetype.
And, somehow, it was everywhere I've gone. I know it was the same one, as it stopped appearing once I killed it, but there were no tunnels where it was appearing/disappearing. And I only killed it before it started spawn camping me due to two massive strokes of luck.