请问大神，为什么3D图跑出来中间缺一条线

代码在这
a=1;k=0.97;c=0.5;Overscript[s, _]=0.8;ce=0.2;
t11=(16 a+b (-2+2 ce+\[Eta])^2+4 b k Overscript[s, _] (-4 c-ce (-2+2 ce+\[Eta])+ce^2 k Overscript[s, _]))^2/(4096 b);
t12=(-2 b (-1+ce)^3+27 a \[Eta]+b k Overscript[s, _] (6 (-1+ce)^2 ce-27 c \[Eta]+2 ce^2 k Overscript[s, _] (3-3 ce+ce k Overscript[s, _])))^2/(11664 b \[Eta]^2);
t1=Piecewise[{{t11,1/4 (2+2 ce-\[Eta]-2 ce k Overscript[s, _])+\[Eta]<1},{t12,1/4 (2+2 ce-\[Eta]-2 ce k Overscript[s, _])+\[Eta]>=1}}];
Plot3D[t1,{b,0,10},{\[Eta],0,10}]