this recently happened to me on two different brands of mono. I would snag a tree or something on the bottom and break off and the line wouldn’t snap at the knot or close to it but would snap halfway losing quite a bit of line. I was fishing 6lb trilene xl and 12lb big game. And yes I have checked rod guides no I’m using an improved clinch knot

   I'd say that means you're tying a good knot    jj

Sounds like bad luck - putting too much strain on the line trying to free the lure. Any line will fail at any point along it's length if you put it under too much stress.

Yeah happy to Say I’ve never lost a fish due to line failure or knot failure except one time 

 

That means it was a spot in the line weaker than where the knot is. This is not unusual. Every like bit of cumulative wear and tear on that spot ranging from small overruns (including tangling and untangleing) to that section of the getting scarped or nicked is the culprit and just part of the game. 

 

I agree that it means you’re tying a good knot, lol.

weakest link in the chain