Thanks so much for your replies!
I hadn’t grounded the extruder motor casing or hotends yet, seems I missed that detail when I first built the printer. Looking back, it totally makes sense to ground those parts 
. It does make me wonder what’s changed recently that’s now causing ESD issues, when everything was fine before.
For now, I’ve attached a grounding wire to one of the casing screws on the back of the extruder stepper motor. I haven’t had time yet to ground the hotend properly in a non-janky way, so I will do that later (or when the issue remains I guess)
I’ll be printing the same model a few more times, so I’ll see if grounding just the stepper resolves the issue and report back.
Thanks again for the help 