Seeing some problems

It's a deficiency - calcium to be precise - and is pretty typical of plants in early flower. You'll need to make sure your pH stays where you have it or even a bit lower... 5.8 is good. You say your ppm has dropped - did you see a corresponding issue with the pH - did it rise or fall? There is a correlation between the two measurements that will indicate what adjustments you need to make to your reservoir... but my guess here is that they are pretty hungry and you need to raise the ppm. I track the EC (using ppm in my equation) because its movement isn't as sensitive as just relying on ppm which can move when you blink... and minor movements of ppm may not be anything to react to... to get the EC, take your ppm, divide it by 1000 and then multiply that by 2... (ppm/1000)*2. I also use a flowchart I've found that will help me understand what I should do to my reservoir given a particular relationship in the movement of the EC and pH (and this also depends on whether or not your plant is drinking)... you can find both charts (drinking and not drinking) here: https://percysgrowroom.com/ec-and-ph-fluctuations-in-hydroponics/ - scroll down, you'll find them. Good luck...