Unusual wear on blade w/Zoysia

I have a lawnboy 21" Goldseries 2-cycle and after 4 cuts I swap blades and sharpen. From 2" of the tip of the blade end the blade they always taper in for about 3" long. This always makes it more time consuming to sharpen the blade to have to smooth that out. I have the mulch kit on it as well and haven't tried taking it off.

I have 3 blades and they all do it. Any ideas why this mower blade would do that?