Hey bud!
This is what I call a "Big risk, Low reward" situation, but that's only if I am understanding correctly. You spraying a nutrient, not a pesticide or fungicide?
Stuff like Bio-Sulphur, Spinosad and Potassium Bicarbonate is to curb mold spores, also known to be the least harmful out of all products to spray during flower, so spraying that stuff in a situation where you feel or know you may have mold pressure during flower will make a lot more sense than doing foliar feed during that time, it may mean more harm than good so it's a bit counterintuitive if there's not significant improvement but big margin for error.
This is actually the more important part here, this needs to be the other way around, you gotta wait for lights to go off before spraying anything. You can damage your plants even during veg when doing this.
There's quite a long list of reasons why and I'd love to explain, but I can go on for days about this so I'll leave it up to you, if you want me to explain I'm open
I agree with Evan, once you got loads of hairs you stop any spray.
It's not the as much the RH that's the problem, though it's definitly a problem too, it's more about the stuff inside whatever you spray.
Spraying clean water during flower can be a problem, because of RH, especially late flower when the buds have internal moisture, but once you mix stuff into that water it doubles the risk, because even the stuff that's safe to spray during flower will burn the crap out those pistils if you mixed it juuuuuust a little too strong or spray at the wrong time
Some people spray bio-sulpher or spinosad during flower, even late flower, without problems, but then you really gotta know what you doing when you gotta spray, how much as dosage and and and..... though with all that said you definitly wana keep your RH more constant and lower during flower, even if you not spraying anything. 30% ideal RH in flower.
IPM is a big must, for every grower in every situation, foliar feed is just extras, but even with IPM -something that MUST be done weekly- as soon as you switch to 12/12 you gotta step back. you must give 7 days between foliar treatments - so lets say you switch to 12/12 today you can do your last full dose spray, next week you'll see one or two pistils here and there when you IPM you do a 50 to 70% recommended dose, another week later if all goes well you'll have pompoms forming and you should spray anything less than 50% dose. 3 weeks into flower you'll have full pompoms and first swelling of calyxes, this is where it becomes dangerous to spray anything and it's completely up you, the general consensus is to not spray anything beyond that point, so if you do and something goes wrong you gotta take it on yourself and remind yourself no one told you to do it and that the rule of thumb is to not do it