Quote - I just would like to undertstand when it will change it to zero, and when it will change it ot the multiple of 12.  The next time it goes up I'm just going to leave it alone, as I can't seem to get it to hold a value above the material's property. 

If it is indeed working correctly, it is certainly not very intuitive.

I've never been able to figure out when and why it will change bounds to zero.

Just take a figure with multiple body parts and multiple material zones.
Usually a material zone covers multiple body parts and a body part may have different material zones.
Now each material may have a different displacement value, each body part could have a different bounds setting.
Also, there is the bounds value from your render settings.
And everything may be related to bucket size which is another value that may change during rendering.

Finally, there may be a bug relating to units set in preferences.
 

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