After doing some searching around it seems that unfortunately the ph level milestone is bugged.

The milestone and wonders page is actually tracking the highest ph instead of the lowest ph. I can confirm this with my own experience, my lowest ph was 3.6, I scanned another animal and got a message saying new record low but the ph level went UP to 4.2. I then scanned another animal and got the message again saying new record low and the ph level went up again to 4.3.

The only way to get this milestone done at the moment is to make sure that the very first animal you scan on a toxic planet has a ph below 2.5, otherwise you’re screwed. Hopefully they’ll patch this at some point.

I’ve seen ppl encountering this bug on xbox, PlayStation and PC, don’t know about switch but I would assume they have the bug too. Apparently if you’re on PC you can use a save editor to reset the wonder and then go to a planet where you know there is an animal with a ph below 2.5 and scan it first. Not tried that myself as I’m on Xbox.