Bug in `MinimalGeneratingSet`

[hulpke]

Extracted from an example created by L. Soicher. Create a group (Order 3888)

f:=function()
local g1,g2,g3,g4,g5,g6,g7,g8,g9,r,f,g,rws,x;
f:=FreeGroup(IsSyllableWordsFamily,9);
g:=GeneratorsOfGroup(f);
g1:=g[1];g2:=g[2];g3:=g[3];g4:=g[4];g5:=g[5];
g6:=g[6];g7:=g[7];g8:=g[8];g9:=g[9];
rws:=SingleCollector(f,[ 2, 2, 2, 2, 3, 3, 3, 3, 3 ]);
r:=[];
for x in r do SetPower(rws,x[1],x[2]);od;
r:=[[5,1,g5],[9,1,g9],[5,2,g5],[6,2,g6],[7,2,g7],[8,2,g8],[9,2,g9],
[6,3,g6],[7,3,g7],[8,3,g8],[5,4,g5],[6,4,g6],[7,4,g7],[8,4,g8],[9,4,g9],];
for x in r do SetCommutator(rws,x[1],x[2],x[3]);od;
return GroupByRwsNC(rws);
end;
a:=f();

Then

gap> m:=MinimalGeneratingSet(a);;Length(m);
5
gap> Index(a,SubgroupNC(a,m{[1,3,4,5]}));
1