I'm using version 11.3
Expand[(Sqrt[2] - t)^2]
2 - 2 Sqrt[2] t + t^2,
but
Factor[2 - 2 Sqrt[2] t + t^2]
2 - 2 Sqrt[2] t + t^2.
On the other hand, without the $\sqrt{2}$ the expression is factored as expected.
Factor[1 - 2 t + t^2]
(-1 + t)^2