Inconsistent boundary and initial conditions when solving sine Gordon equation

I have been trying to solve the following sine Gordon equation

$partial_{x,x} u(x,t) – partial_{t,t} u(x,t) – sin (u(x,t)) – alpha partial_t u(x,t) + gamma = 0$,

for $x in (0,15)$ and $t in (0,20)$ with the boundary conditions

$partial_x u(x,t)vert_{x=0} = h, hspace{2cm} partial_x u(x,t)vert_{x=15} = h + a_{mathrm{ext}} sin(omega_{mathrm{ext}}t), $

and the initial conditions

$partial_t u(x,t)vert_{t=0} = 0, hspace{2cm} u(x,t)vert_{t=0} = h x$.

Here is my Mathematica code

const = {al -> 0.08, (Gamma) -> 0.01, h -> 6,  ax -> 2.5, (Omega)x -> 1.4};

NDSolveValue(({D(u(x, t), x, x) - D(u(x, t), t, t) - Sin(u(x, t)) - al D(u(x, t), t) + (Gamma) == 0,
    
(D(u(x, t), x) /. {x -> 0}) == h, (D(u(x, t), x) /. {x -> 15}) == h +  ax Sin((Omega)x  t), 
    u(x, 0) == h*x, (D(u(x, t), t) /. {t -> 0}) == 0} /.const), u, {x, 0, 15}, {t, 0, 20},
 
 Method -> {"PDEDiscretization" -> {"MethodOfLines", 
     "SpatialDiscretization" -> {"TensorProductGrid", 
       "MinPoints" -> 200}}})

The code works and gives a solution, but there appears an error message like this

NDSolveValue::ibcinc: Warning: boundary and initial conditions are inconsistent.

From the second initial condition we have $partial_x u(x,0) = h$, which are consistent with the two boundary conditions at $t=0$. So I don’t understand why the above error occurs.

I have read many posts related to this problem. One of the solutions is to increase the “MinPoints” in the “Method”. I tried so, but it still did not not get rid of the error.

Any advice is very much appreciated.

Dat.