It sounds like the blade is well beyond its usefulness, as I go right through SECURE knots with no problems, loose knots should be avoided. Blades are only $30-50. Even with a dull blade I cannot see it catching a knot and bringing the board upwards unless its startled you and you released the mitersaw handle, thus allowing the spring to bring the board upwards.
With a dull blade I have got a lot of burning and excessive motor strain.
Not trying to be argumentative, just trying to help. Whenever I'm working on the miter saw I always do the following:
-Make sure there is no wood pieces that are keeping the stock being cut either not level with the bed and not tight against the fence.
-Make sure the stock is supported well on both sides of the bed so that it doesn't pinch the blade.
-Grasp the back side of the fence with my four fingers and the stock with my thumb so there is a firm grip and control, so that my hand doesn't go where I don't want it to.
-With firm, slow pressure and the saw at its operating RPM, plunge the saw into the wood.
-If there is any concern during the cut it is always on my mind to let go of the trigger, but keep down pressure on the swing arm of the saw.
I hope these things help you.