Now I can only assume.
I assume you had a stable system running at this 1000ppm prior to the drops your seeing.
I will assume this would be 2ec.
When I change a substantial amount of water , like say all my 40 gallons and I do monitor my numbers .
Kind of ocd.
I will see ph drops .5 every 12hrs for couple days and then stops completely.
I don’t run co2 , so en ec1.6-1.8 would be about as high as I would ever go.
1.4 ec is a balancing number ( ec and ph) for me at this present time.
So if I was to see my ph drop like you and it was once stable ?
I would think I have something in my roots.
Like a mound of roots that look to have some browning inside.
I would massage this brown area underwater that it’s in and then maybe add more beneficials to fight the problem.
Now if your system has never been stable, ( stable is were ph stays the same and ec too. )
I would think your ec is too high.
My
Enjoy and good luck with your grow.