Well, I WAS going to mention the fact that GVT is NOT something to do if you expect to keep current max strength levels. The intensity/volume is too low/high, respectively. I was going to say that you should focus on max strength with low reps/volume for a while after your GVT volume kick. That will bring you back up. Remember, you get what you train for, so if you drop the intensity all the way down in favor of massive volume, max strength almost always drops.
Same thing on the other hand: if you train low volume/high intensity, your strength will go up, but your muscles' endurance/metabolic efficency at a given %1RM of your current max will drop. (Not to be confused with metabolic efficiency/activity in diet. I mean your muscles' ability to handle cellular fuel resources efficiently and hence a % weight for a longer period of time).
Also shift your calories around (timing, not quantity). Eat a bigger breakfast.
Now I'm not sure. But I think I'll stick to my advice. It seems your CNS wants to give out early, whereas your muscular endurance is fine from the GVT, as you don't "feel" anything.
Just some thoughts. Hope this helps.