Any ideas as to what's causing this

Also if course it’s not every bag of FF, the huge majority love it and have no issues. That said a quick search shows there are lots and lots of issues with PH.

UGH!!! That was a typo!!! I meant lower N and up PK… SORRY!

Not seeing anything you are talking about. Here are the top 10 soils for 2022. OF and HF are 1 & 2.
https://gardenguidepost.com/best-soil-for-cannabis/#:~:text=The%20best%20potting%20soil%20for,also%20the%20most%20expensive%20type.
This is from a user prior to buying the product. Perfect pH to start.
What is the pH level of the Happy Frog potting soil?
6.3-6.8 Ph

There are hundreds in this forum alone, I just replied to 3 yesterday that are recent post but the forum is littered with it. Rollitup, thc farmer, FB groups, local stores that sell it, literally they are everywhere if you Google. Low PH being low as in low 5s. Most people myself included put in 6.5 or so and out is 5.2 give or take that’s the common theme. Others check the PH out of the bag and it’s high 4s and low 5s.

Copy that, I guess I am just one of the few lucky ones that have never had an issue. Yay me!!