SageMath version 9.3, Release Date: 2021-05-09
on Mac OS, but the bug I encountered also occurred on a Windows machine.
R = RootSystem(['D',4,1])
L = R.weight_lattice(extended = True)
leads to a long error message, the final line of which is
TypeError: unsupported operand parent(s) for *: 'Rational Field' and 'Extended weight lattice of the Root system of type ['D', 4, 1]'
Note that the positive root  is the first one at grade 1.
One can get around the bug by accessing the positive roots in L first, but it would still be nice if this could be fixed. Thanks.