A tuple of one type + a TypeVarTuple can't be assigned to a bare tuple:
(In Pyright Playground)

def accepts_any_tuple(t: tuple):
    ...

def test[*_Ts](t: tuple[int, *_Ts]):
    accepts_any_tuple(t)

Argument of type "tuple[int, *_Ts@test]" cannot be assigned to parameter "t" of type "tuple[Unknown, ...]" in function "accepts_any_tuple"
  "tuple[int, *_Ts@test]" is incompatible with "tuple[Unknown, ...]"
    Tuple size mismatch; expected 1 but received 2

It looks like the tuple is inferred to have size 2 for some reason?