Oh? That might blow my idea apart.
Is that a balanced mic and input, and XLR to XLR cable?
It seems odd, doesn't it? I understand the full chain is balanced. I'm happy to spin up a new post if it's more appropriate before launching into fresh discussion. What struck me about OP's post is the very close similarities of our problem. The setup I have is:
Interface: Focusrite Scarlett 18i8 3rd Gen, Audient iD4.
Issue occurs with: Shure SM57, SM58, Fender Stratocaster.
But not with: Rode NT1 5th Generation on phantom power.
Cables (both tested): XLR to XLR Nuetrik NC-MXX connectors, Sommer SC Club Series MKII (cable 1, 3m), Rode Professional (cable 2, 5m).
Laptop: MacBook Pro M1 Max.
Audio sample from Focusrite attached (occurs when monitoring and recording), first two tones recorded simultaneously.
Tone 1: SM57
Tone 2: SM58
Tone 3: Stratocaster (humbuck pickup near bridge, balanced TRS)
So the SM57 and SM58 pick up the same signal, but when I had the Rode and SM58 plugged into two different XLR inputs, the SM58 would pick up the signal, but the Rode would not.
I moved into a new flat, purchased the dynamic mics (my first dynamics) and Focusrite all at the same time, so I had three new variables to deal with.
The furthest I have gone to date to isolate the cause(s) was to run the Audient iD4 with the MacBook and no other peripherals, no bluetooth, and no wifi, in another room, turning off the entire mains power in my flat (just long enough that the freezer wouldn't defrost

). The issue was prevalent to the same degree in that second room. And walking around the room, changing the position of the mics/guitar did not affect the strength of the signal. Pulling the cable from the interface killed the signal immediately, but I don't believe I tried pulling the cable out of the microphone/guitar. I took away from the testing that the problem certainly seems to originate from the similar and more sensitive components of the guitar/microphones picking up a strong external signal, and are frustratingly most easily triggered by producing sound, even by an acoustic guitar that's not plugged into anything.
My next steps:
1) Do a field test at a friend's house.
2) I've ordered a Gotham GAC-3 with Neutrik EMI XLR ends so I have a RFI-resilient cable to add to the chain. Doesn't hurt to have this regardless.