Is the PH spike from adding Armor Si (or the like)...

It might as it runs out, but why would you knowingly run your PH out of your target zone? Check daily and adjust, then the PH on the Silica won't really matter. You should never add any nutrient directly to your reservoir, but Silica especially. Silica needs to sit for about 15 min before you should add other nutrients to the water. Read the bottle, add your nutrients in the proper order, PH 5.5-6.2, and you will crush it. Silica -> Cal-mag -> Micro -> Grow/Bloom -> Beneficial's (if you use em) This site breaks down the nutrients and provides a lot of good information on how your PH could fluctuate depending on what you feed that plant. https://www.dudegrows.com/mystros-guide-to-cannabis-nutrition/

I’m in hydro and was interested in some insight to a common ph issue found using various nutes. If you ph goes up great, I was looking for some people who experience the same, and can provide some logic and science. If we can do that, please pass along an answer.

Ph rises when adding Si, but curious if that tails off over time. This question is independent of rising ph in general hydro chemistry.

I’ve never had a drift down, only up…

Yes it can definitely drift, just after a week of use check your soils ph and you'll have a idea