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seruous solve bug? only when used with simplifying

hello group, 

i think i have a serious Solve[], Simplify[], FullSimplify[] bug...

Take a system of equations as below. (apb is actually (a+b). i defined
it as apb to keep Mathematica crunching away at it)

sys = 
{c1[t] == (c*(16*a^4 + 8*a^3*kd + 4*a^2*apb*kd + 2*a*apb^2*kd +
c2[t] == (2*(8*a^3 + 4*a^2*apb + 2*a*apb^2 + apb^3)*b*c)/(apb^4*d), 
c3[t] == (2*(8*a^3 + 4*a^2*apb + 2*a*apb^2 + apb^3)*b*c)/(apb^4*kd)}

sys1 = sys /.{apb -> (a + b)}
sys2 = sys/.{apb -> (a + b)}//Simplify
sys3 = sys/.{apb -> (a + b)}//FullSimplify

and then use

TrueQ[sys1 == sys1]
TrueQ[sys1 == sys2]
TrueQ[sys1 == sys3]
TrueQ[sys2 == sys3]

then it gives, 


Simplify and FullSimplify changed the system so much that they are not

Basically, unless I define the (a+b) as a single constant apb, the Mathematica
will do soemthing with it then it messes it up. I think...

Why is this happening? and how do i prevent it from happening again
with other more obsucre systems?  Above I got lucky in finding the
cause (a+b)

thanks in advance for any and all comments. 


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