Simplification depends on the names of variables

I am sure this issue has been discussed here, but it has never been asked whether this behavior is a bug.
Therefore my direct question to the community: do you think this behavior is a bug?

poly = a1 b1 e1 + a2 b1 e1 + a3 b1 e1 + a1 b2 e1 + a2 b2 e1 + a3 b2 e1 + b1 c1 x + c1 d1 x + c1 x z;

FullSimplify[poly]
Out[1]= a3 b1 e1 + a3 b2 e1 + a1 (b1 + b2) e1 + a2 (b1 + b2) e1 + b1 c1 x + c1 d1 x + c1 x z

FullSimplify[poly /. x -> a]
Out[2]= (a1 + a2 + a3) (b1 + b2) e1 + a c1 (b1 + d1 + z) 

The LeafCount is 33 and 17, respectively. How one can achieve a consistent simplification independent on the names of variables?