Cash is king!
So she took about 4 Gallons before hyper run off. Had a strip test left and you’re right @Laughing Grass looks like 4.5 ish.Since you're not in flower this is more likely a lockout than a deficiency. Sweet spot for phosphorus uptake is between PH 6-7. Promix is acidic so I suspect your rootzone ph is a little low. Do you water to run off? When you get your pen, water to get enough run off and test. If it's super low... 4-5 I'd water at a PH of 7 to bring it back into range.
Personally if I were to ever run promix again I'd do my first couple watering at 7 just to bump it up a bit.
This is so early that you can solve this long before it goes into flower.
Disclaimer: True at the denominations I deal in. Some of you billionaires may not feel that way with your freezer driers and such.Cash is king!

Let's do the time warp agaaaaaaain.
It's just a jump to the left.....Let's do the time warp agaaaaaaain.
Morning.
Sugar and spice and everything nice.
Growing up with three sisters in the tribe ... nope!!Sugar and spice and everything nice.
She looks high.Looks like she fought off a tiger!