My girl has a single drown leave on her, the ph has been stable, but she has been drinking about 2 litres every 6 hours, the ppm spikes n gets brought with fresh water, not letting it get above 900.
If your ph drops there is to high of an ec level cuz they drink more water than food. When your dialed in, they drink and eat pretty evenly. Ph keeps dropping because they drink to much water and if your ph rises they don't get enough food so they need higher ec. I would say with autos your ec is to high. Try 650 ppm 1.3 ec. See if that stabilizes your ph, but prepare the water 24 hours in advance to let the ph settle out before nuting and ph ing your water.
I wouldn’t worry. That looks like it got burned or something with heat maybe or something was on the surface and burned it.
Also, I’d cut that EC/PPM a down a bit.