IMO it is a "pinch" due to the knot compressing, pick out the knot and plug it. Hopefully it won't get worse.
If you leave it, it will probably get worse. Even if that's not the cause, plugging it will reinforce that area.
A knot may be solid, but is often surrounded by black manky bark which has zero compressive strength, the bark will collapse stressing the surrounding wood.
If the knot is an actual hole, that has zero compressive strength too... maybe replace it with a hole from some Osage... they are much stronger
Del