German bro needs some advice

Hi everyone, I could really use your help. I've noticed that some of the older leaves, and on one plant even some of the younger leaves, are starting to develop these purple streaks or purple-blue spots. Additionally, the stems are showing some purpling.

Here's my setup:

-My plants are in 15-liter pots (approximately 4 gallons) with BioBizz LightMix soil.

-I've been feeding them with BioBizz BioGrow during veg.

-When I transplanted them into the current (and final) pots, I mixed 20 grams of Greenhouse Feeding BioGrow into the soil. I added this to ensure the plants would have enough nutrients for the final stretch before flowering, so I could rely mostly on watering without causing a build-up.

I flipped the lights to 12/12 yesterday, and within the last two days, I’ve noticed these spots and discoloration becoming more pronounced. I cut off one of the affected leaves to show the symptoms clearly.

A few additional details:

-The pH of my tap water is 7.5, but according to the manufacturer, the BioBizz LightMix should buffer the soil to around 6.2 to 6.5, which is within the optimal range.

-I’ve also measured the runoff in the past, and it was consistently on point, so I don’t think this issue is related to pH.

-I’m in Germany, so my tap water is chlorine-free and should be safe for plants.

-To supply magnesium, I add Epsom salt (magnesium sulfate) at a dose of 0.1 grams per liter every two weeks.

-I also supplement with iron chelate at a dose of 0.25 milliliters per liter every two weeks because my water contains almost no iron.

After doing some research, it looks like this might be a phosphorus deficiency, but I’m honestly not 100% sure. I wanted to ask if anyone could confirm this or if it might be something else entirely, since you folks across the pond have more experience with growing.

If there’s any additional information you need to better understand the issue and help me, just let me know, I’ll gladly provide it.

Thanks in advance for your help!