Wrong stabiliser in integral action

[stertooy]

ConjugacyElementsBySeries can return an incorrect conjugating element:

gap> G := ExamplesOfSomePcpGroups( 10 );;
gap> g := G.1^2*G.3^3*G.4^-3;;
gap> h := g^(G.1*G.2^2);;
gap> pcps := PcpsOfEfaSeries( G );;
gap> k := ConjugacyElementsBySeries( G, g, h, pcps );
g2^3
gap> g^k = h;
false

With the help of one of the debug variables, I get this:

gap> CHECK_INTSTAB@Polycyclic := true;;
gap> k := ConjugacyElementsBySeries( G, g, h, pcps );
Error, wrong stab in integral action at /usr/lib/gap/pkg/polycyclic/gap/action/orbstab.gi:651 called from
OrbitIntegralAction( Image( nat ), act, e, f ) at /usr/lib/gap/pkg/polycyclic/gap/pcpgrp/centcon.gi:302 called from
<function "ConjugacyElementsBySeries">( <arguments> )

I can recreate the bug all the way back to v2.11, and it doesn't seem to get fixed by merging any of the open PR's, sadly. It also seems to be independent of #58 and #70.

Perhaps interesting: g and h both lie in a subgroup of index 3, and the bug doesn't happen in that subgroup:

gap> S := SubgroupByIgs( G, [ G.1, G.2, G.3^3, G.4 ] );;
gap> pcpsS := PcpsOfEfaSeries( S );;
gap> l := ConjugacyElementsBySeries( S, g, h, pcpsS );;
gap> g^l = h;
true