I contracted the spraying out to a large, well-established and respected outfit. I'm confident they use fresh product.I would have thought that rate would’ve killed it. If it were stressed from drought, all bets would be off. Otherwise, I would’ve expected it to work. You are using relatively new glyphosate?
I've dealt with small patches of Johnson grass in the past. This is NOT Johnson grass. I am certain about that. I can see Johnson grass in my sleep. The rhizomes of this weed are much smaller, and the leaves are more delicate.In one picture, it looks like Johnson grass.
Johnson grass is highly invasive and EXTREMELY difficult to kill. It is one of the plants that grows from rhysomes, seeds, and tillers. Disking or plowing causes it to double/triple because it only takes about an inch of the root to start a new plant.
The mistake most people make went spraying Johnson grass is to go too heavy. Too heavy and it kills the top of the plant but leaves many root segments alive and you end up with more plants. One recommendation, which I follow myself, is to use a half rate which gets more of the roots.
Around here, it is spread mostly by the road districts who mow the roadsides after it has gone to seed but not yet dropped. It's a major problem hereabouts.
Johnson grass also produces cyanide when stressed, making it poisonous to livestock.
If I was to design the perfect plant--it would be Johnson grass.