87 is really high mate, I personnaly don’t like anything over 78 at worse. I like keeping it 75-73. Different genetics mean different taking care of, it has to be taken into account that some may be more resistant then other, vice versa. I like multiple genetics grow for the personal experience gain over growing 1-2 strain... but, it’s more hard to figure when an issue happen, some plants react less and some more. If you change something, it might affect the one that are fine and still yet don’t see result on the one you wanted to fix. You have to get a log of witch strain suits you better for your environnement for future personal reference! The PH shouldn’t change that much, except , PH change with the temperature, so if you put water at 64f and it goes out at 78f, it could explain the rise, you need to counter balance this effect by adding water at good temperature. If within the same temperature you change .2 .3, it could be a PH lockdown and need to be look into. Sometime the meter need a little calibration too. First fix your environnement temperature, second, flush and watch for PH change. You have time to fix before flowering, but 6 plant in 4x4, they are gonna double size in flower so maybe you want to clear a little space in there ( defoliation ) it’s gonna help them breathe a little :p good luck mate