The answer is in dec2bas(x,b).
In the third row there is x

x-m)/b; arguent:something inside the function is a pass-by-reference operation from the function to the argument, hence the behavior (900 becomes 0).
If you don't want it, assign x to an internal variable and do all the definition operations on that variable (f.e. z:x -> z

z-m)/b)
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