Not having a problem - guitar sounds fine, so this isn't terribly important, but I'd like to understand, so...
I read a lot of stuff about avoiding ground loops and I just don't get it so I will pose a (probably) ridiculous question in the hopes that somebody will set me straight.
I recently bought a shielding kit from stew mac and shielded a jackson guitar - shielding tape in the pickup cavities, control cavity, control cavity cover.
Lug screwed into each pickup cavity, with wires running from each to a lug that is also attached to the side of the control cavity. There's a wire running from the control cavity lug to the volume pot case.
To me, this sounds like a ground loop because the volume pot case already touches the shielding tape by virtue of the fact that it sits right on top of it and is pushed through the hole to the face of the guitar. So, there's the lug with a wire connecting the shielding to the pot and the shielding touching the pot directly.
There are no ridiculous questions if one doesn't know the answer already. There are, however, ridiculous answers around . . .
ChrisK discusses this point about ground loops in his thread on The Blocking Capacitor. Read down the page about 2/3 of the way.
The point to take away is that the only ground loops that can possibly induce noise into the circuit are those that are carrying signal. Your example is a loop, to be sure, but it's only a loop through the shielding, not in the signal path of the circuit.
And, while a lengthy ground loop might induce some noise, short ones are probably OK anyway.
While these signal return jumpers are in parallel with the shield (which is a shield AND NOT a reliable signal return path) they are not functioning ground loops until one gets into the RF (radio) part of the spectrum.
I don't think that these are ever a problem. Folks recommend removing them because they don't understand the rational behind them, and have heard that "ground loops are bad", as they generally ARE NOT electrical engineers.
The effective grounding in a modern Stratocaster. makes use of all three pot back shells being jumpered together. This costs money to do and Fender makes a lot of Stratocasters, but they feel that this is an important step.
There is a huge ground loop in electric guitars; it's called the cable shield to the amp and the operator to the floor. Just because a concrete slab is poured on the ground, it is not necessarily a low impedance path to the copper stake safety ground at the power panel input to the building.