NDSolve returns different answer for different value of xmax

Question

I have solved a differential equation by using NDSolve with xmax=3

ydum = x/L[t]; 
expr1 = Expand[(1/c^2)*D[\[Psi][ydum, t], {t, 2}] - 
     D[\[Psi][ydum, t], {x, 2}] + ((m^2*c^2)/h^2)*\[Psi][ydum, 
       t] /. \[Psi][ydum, t] -> \[Psi][y, t] /. x -> y*L[t]]
m = 1; 
c = 1; 
h = 1; 
\[Omega] = 1; 
L[t_] := 2 + Sin[\[Omega]*t];
sol = NDSolve[{expr1 == 0, [Psi][0, t] == 0, [Psi][1, t] == 
    0, [Psi][y, 0] == 
    Sqrt[2 (m c)/(c Sqrt[m^2 c^2 + [Pi]^2 h^2/L[0]^2] L[0])]
      Sin[ [Pi] y]}, [Psi], {y, 0, 1}, {t, 0, 3}]

Then I plot this with Plot3D

Plot3D[Abs[Evaluate[\[Psi][y, t] /. sol]], {y, 0, 1}, {t, 0, 10}, 
 AxesLabel -> {y, t, \[Psi]}]

Then I changed the xmax of my NDSolve to 10 and plot the answer still with t ranging from 0 to 3

sol = NDSolve[{expr1 == 0, \[Psi][0, t] == 0, \[Psi][1, t] == 
        0, \[Psi][y, 0] == 
        Sqrt[2 (m c)/(c Sqrt[m^2 c^2 + \[Pi]^2 h^2/L[0]^2] L[0])]
          Sin[ \[Pi] y]}, \[Psi], {y, 0, 1}, {t, 0, 10}]

The 2 graphs are clearly different. From my understanding, changing the range in which a differential equation is evaluated shouldn't change the answer (I have also tried it with simpler differential equation and this didn't happen). Can someone explain why this is happening?