Hi,
I had this problem and I use the loop in a 4cm method with a Marshall VM, wanted to just add 2 quick solutions.
1. Sticky tape your cables outer jacks ring (thin layer of tape obviously so cable can still go into socket) thats going to your amps fx send and return.
2. More permanent pro solution is get a 4cm junction box for your pedal board. These usually separate and disconnect the grounds from the fx send and return part of the circuit.
(this also has the benefit of switching to regular 2cm if you dont want to use 4cm by just pulling out a cable and then internal switching runs all pedals now before your preamp)
best solutions i found:
https://brightonion.co.uk/products/shifter-switch-patchbox-v2
and vertex has a version with and without buffers (but they stupidly expensive for they are but yeah he has a video on how to make one yourself)
Hope this helps someone.