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That's because B.coord() returns a tuple, so that the multiplication t*B.coord() is interpreted as a tuple multiplication, which has meaning only if t is an integer, with a result different from what you had in mind:

sage: B.coord()
(-1, 2)
sage: 3*B.coord()
(-1, 2, -1, 2, -1, 2)

So a solution is

sage: C_1 = R1.diff_map(R2, [t*B.coord()[i] + (1-t)*A.coord()[i] for i in range(2)])
sage: C_1.display()
R1 --> R2
   t |--> (x, y) = (-4*t + 3, 2)