I know I'm a little late to the party. How's your grow going? I think the issue you experienced was similar to my own...FFOF feeds for a while but I'm betting you waited too long to give them their first feed. That's what I figure I experienced. When you can visually see most deficiencies, especially soil grows, the deficiency has already been present for a while. Just out of curiosity, do you happen to remember exactly how long it was after their first feed that you started seeing problems?
I waited 6 weeks until my first feed, but I was still in veg at that point, and that was too long. 7 Gallon containers. My girls did what yours did after I fed them, continued to lose the leaves already affected, but all new growth and unaffected current growth was fine. Next grow I'm going to start feeding them a lot earlier in smaller amounts. Your issue was probably compounded by the fact that your plants were going through their flower stretch. They require a good deal of nutrients to support all that fast growth.
Additionally, I spoke with a FF agent a few weeks ago and she specifically said that FFOF tends to lack potassium towards the end of 4 weeks, which is compatible with your suspicions of there being a K deficiency. If you want to post an update I'd appreciate it, as we're growing in similar conditions.
What's the deal with that one bud you mentioned that looks like it has a super high leaf to bud ratio? I was thinking it could be early foxtailing but that is just a complete guess and also based off a few of the leaf tips looked light burned.
Cheers