"Use Item" not working while sneaking and "Use Item" bound to NUMPAD0

I changed my controls to a configuration more comfortable for me. My new key map is as follows:
Use Item/Place Block = NUMPAD0
Sneak = RSHIFT

With this combination I am unable to place signs on specific items (e.g. place a sign on a chest). When I hold down the "sneak" key and press the "use item" key, the chest opens instead of placing the sign on the chest. If I want to place a sign on a chest, I have to change my "Use Item" key to PAGE DOWN, then change it back to NUMPAD0 when I'm done.

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Code analysis by Marcono1234 can be found in this comment