Coolant loss- any connection to a fuel injector

Last week I was tooling along with my 2014 at 354,000 miles. all of a sudden it consistently started the death rattle, and it eased up pretty quick. But it was consistent every time. I use that app called scanner app, and it never gave a count of misfires, but the only data it did pull was that they occurred around 1300 RPM and although the check engine light didn’t come on, if you read the codes, the P301 was there.

I did the head gasket replacement about 100,000 miles ago, new OEM spark plugs, clean the manifold ports, EGR cooler.

I got to thinking about the advice to do a fuel system cleaning or at least just run some good Chevron fuel system cleaner.

I followed the instructions on that, actually ran my gas tank to about as empty as it could possibly be,… Not on purpose it just happened that way. I ran into Walmart with 0 miles left to go.

Anyway, the death rattle has stopped. Coincidence, just luck? doesn’t matter either way I was still pretty happy… Until I noticed that it looks like 3/4 inch of antifreeze has gone down in the reservoir.

What would be the next order of inspection or replacement? I’m thinking probably spark plugs would be the next place to go. And then the EGR cooler. any thoughts?