If I queue it up like you suggested, the last two plants do not actually get fertilized nor do the items get removed from my sims inventory. If I click on the plant to try and fertilize it again, it is grayed out. So it looks like only the first item of a stack gets used successfully if trying to queue up several plants to fertilize at once.
For myself, and it's been this way for me before Seasons, I always queue up 7 plants to fertilize with the top item (paused game then live once all 7 are queued). Always, it uses one item for those 7 plants. I always considered this a perk as you use less fertilizer this way. All of my plants benefit from being fertilized. I just checked all of my Seasons game plants and they are all at the same quality growth level. So for me, I see no change to how it's always worked.
