Grinding belt makes concave edge at the heel

[MaxtheKnife]

I found my Viel S-5 makes curve at the heel of blade. I think it's because the tension of belt is not even i.e. the tention of outside is lower than in the center. 

How do you prevent it? I think it's helpful to maintain low pressure, but it doesn't seem to be a complete solution.

i was just getting ready to post the exact same thing........for years now i have not had this problem, but last few months with some of my restaurant exchange accounts i notice the same.

i have attached 2 pics for review and comment........
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https://imgur.com/a/aBHfouI

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https://imgur.com/a/aBHfouI