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First, the workaround.

sage: var('y')
y
sage: a = x*y
sage: solve([a,1==1],[x,y])
[[x == r1, y == 0], [x == 0, y == r2]]

There is an open ticket about this I will try to find later. But at least now I know the reason.

This is interesting - apparently we assume that if one passes in a single expression, there is a single variable that should be solved for.

    # There *should* be only one variable in the list, since it is
    # passed from sage.symbolic.relation.solve() and multiple variables
    # there don't call this function.
    if isinstance(x, (list, tuple)):
        x = x[0]

That explains your result. However, @moroplogo's is even more interesting. What happens is that all arguments get passed to xy.solve()

if is_Expression(f): # f is a single expression
    ans = f.solve(*args,**kwds)
    return ans

But these are not unpacked! So we have something that actually passes in to Maxima. But what? It's not passing in this:

(%i2) solve(x*y,[x,y]);
(%o2)                [[x = %r1, y = 0], [x = 0, y = %r2]]

and some debugging indicates it should just be passing in the same as solve(x*y,x). I'm not sure how that extra [1] gets in there.