Sounds like you're dealing with an Anubias nana soap opera here! First things first, these plants are pretty hardy but can be fussy about a couple of things. Like the others have said, these guys aren't sun worshippers and hate direct lighting, so if your LEDs are cranked up, they might be crying out for a shady corner. Try dialing the light down or moving them to a spot where they get some shade.
Next up, check out the rhizome. Anubias hate having their rhizome buried in substrate, so if it's tucked in there nice and snug, it’s on the fast track to rotting city. Make sure it's sitting pretty on top of the substrate or tied to rocks or driftwood.
You're saying water parameters are fine, but make sure the water isn't too warm—these plants like it cooler. No roomies like pesky algae or persistent snails munching away at them, either.
Last but not least, maybe the plants you got weren’t in the best shape from the start. Sometimes they’ve been sitting in unhappy conditions before they even get to you, making them more likely to bite the dust. Good luck, and let's hope your Anubias nana makes a comeback!