Question: Weird behavior of subs and isolate in PDE systems

Hi folks, I'm running Maple 17.02 and getting very different behaviors from subs and isolate. My actual expressions are way more complicated but this simple example illustrates the problem:

restart; with(PDEtools); F := diff_table(f(x, y, w, z, a, b));

subs(F[x] = Z(x, y, w, z, a, b), F[x]+F[x, b]-10);

subs(F[x] = Z(x, y, w, z, a, b), F[x]+F[x, y]-10);

The last line gives me what I want  Z+Z[y]-10 but the second line gives me Z+F[x,b]-10. That is, if I use b instead of y as variable, subs ignores the second derivative...

Can any one replicate this behavior? Is it a bug? Thanks for any help.

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