When you water, it looks as though the water level is rising just enuff to hit those bottom leaves before the water is absorbed.

Looks like all of mine do on the bottom leaves.

Nothing to worry about, I am certain of this

It maters not as those bottom leaves are first to die anyhow. But this will happen every time pH down adjusted water hits those bottom leaves.

All is well.....as long as it doesnt show any higher up, thats what it is and you will be fine.