op amp random pulsing to ground
It can be difficult to find random 1us interference particularly when you don’t know where to look.
When this happened in my early days to me I looked around and saw my temperature controlled Weller soldering iron and with a quiet click sound of a  relay, I said, that’s it! Radiated EMI pulse from a dry contact inductive load during turn off with dt approaching zero except for air gap capacitance.
Your supply filtering has a ridiculous amount of filtering so the cabling might make the difference on ground shift noise, but the line side mass massive inductance.  This of course doesn’t work without some series DCR coil resistance of a few Ohms and with just that R, you don’t need the inductor when your load is on the order of a 1k Ohm. In theory with an ideal L that would produce a Q of 1000 at a couple hundred Hz, which isn’t possible maybe a hundred or so.  but that can’t possibly create a 1us pulse, but if you had some motor noise on the supply, it would ring at that frequency and be amplified by 40dB or so with this high Q series LC circuit. So I would prefer a cost reduction and a 1st order RC filter or CMOS LDO if necessary from unknown noise levels but this can be managed by knowing your PSRR and all your load ripple.