#1 imho, I don't spray anything especially in flower, although the 1st few weeks are generally ok before budset.
I used to, especially for mites, but I try and do all of my spraying in veg, if I can. But in veg,I do several homemade herbal bug sprays and an aloe vera soak/ spray at times too.
I am in the corner, that plants can thrive without being sprayed, that is not to say that I am against foliar spraying as certain things are more advantageous, to each their own
#2 I am a recycled soil grower, but have you checked your KH? If its low, something is being repeated in the pH swing. Are you adding cal/mag along with a bloom nute already with Cal or something alkaline?
I would suspect the KH of the nutes is low and the KH of the tap water is higher. Seems like it stabilized back to the tap water pH overnight. When the KH is low, alkanity is decreased, which is basically the buffering effect of pH rise/drop.
Usually when people have wild pH swings, the KH is low, because the buffering effect of pH change is very low.
Although you have a gradual swing, just after feeding? probably as the feed nutes become diluted? That's my guess.
Adding to something to increase the KH without significantly raising the GH is also a challenge at times, but in a pinch pH UP, might help.
You might just need to soften your tap water, a tad bit too. Ever do a TDS reading on the tap?