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Hint:

sage: ddom(x,y)
abs(x - 0.500000000000000) - 0.500000000000000

This shows that SageMath (or rather, Python) didn't catch your intended meaning when you wrote max. If you want to work with symbolic expressions involving the maximum, then you should use the symbolic maximum:

sage: ddom(x,y)=max_symbolic(abs(x-0.5),abs(y-0.5))-0.5
sage: ddom(x,y)
max(abs(x - 0.500000000000000), abs(y - 0.500000000000000)) - 0.500000000000000
sage: ddom(10,20)
19.0000000000000

If you don't need symbolic expressions, then you can also define a Python function instead:

sage: ddom2 = lambda x,y: max(abs(x-0.5),abs(y-0.5))-0.5
sage: ddom2(10,20)
19.0000000000000