I have not run into that. but let’s take it from the beginning. How are you defining the materail resource?
Typically it is defined with a “Type” of “Material”, “Standard Rate” equal to some value; e.g. $1,000.00 and “Cost/Use” of $.00
Example: I would assign “.4” units, then you would have to decide how to allocate that cost over tinme. In the Task Usage view you could allocate “.1” units per calendar day, allocate the whole amount on the last day or the first day or whatever your process is.
It could be some combination of the resource definition and allocation over time is affecting the result.