bug of function keep_weight

Question

The following code:

{u,v}::Weight(label=field,value=1);
ex:=(u v)**{1/2};
keep_weight(_,$field=1$);

I get the wrong result $0$, while the following code

{u,v}::Weight(label=field,value=1);
ex:=(u v)**{1/2}+u u;
keep_weight(_,$field=1$);

work fine. It seems a bug.