Question: Groebner is having problems with subscripts?

Hi.

I have 7 polynomials r[9], r[6, 6], r[7, 9], r[8, 6], r[9, 6], r[6, 9, 7], r[6, 9, 6] each of 7 variables a[3],...,a[9] and I generate an ideal QQ usinig these polynomials. What Maple tells me quite fast is the follows:

IdealMembership(a[3], QQ) 

true

Also using Groebner as follows
F := [r[9], r[6, 6], r[7, 9], r[8, 6], r[9, 6], r[6, 9, 7], r[6, 9, 6]];
G := Basis(F, tdeg(a[3], a[4], a[5], a[6], a[7], a[8], a[9]));
provides me with the basis quite fast and a[3] is part of it. 

---------------

However, Mathematica is stuck and I have noticed that if I change a[3],...,a[9] to x,y,z,,v,w,t then Maple with the same polynomials is evaluating very long time. 

My question is: what am I doing wrong with subscripts a[3],...,a[9] in Maple? Does Maple 2019 have a problem with it?

I have attached my worksheet. 
Maple_Problem.mw
Would appreciate any help.