So far, I'm not able to replicate the stray line issues here, but my case (some generic flip cover with an elastic band to hold it shut) does not have magnets because I was concerned about the effect on the s-pen behavior so it could be that.
However, in terms of a fix you could try adjusting the sensitivity of your s-pen itself.
CAUTION!! you may damage it by doing so, I'm not responsible etc etc, do at your own risk !! ---
The click button along the side of the s-pen (with the ridges in it) is remove-able and underneath it is a screw you can adjust (in very small increments please) to adjust the sensitivity of the pen. The screw NEAREST the pen tip (there are two screws next to each other) is what you would adjust --- going CLOCKWISE to DECREASE sensitivity, and COUNTER-CLOCKWISE to INCREASE sensitivity.
I adjusted it to be slightly less sensitive as my pen was registering on the screen sometimes without touching it --- it appears that the tail-ended portions of your E's may also be a result of over-sensitivity.
YMMV of course
Samsung Galaxy Note 10.1
Moto G 16GB US GSM