There might be the possibility that infested leaves on maple trees don't give special output at all, neither silkworms nor string, if harvested by crooks. Maybe the silkworms we got when we had the crook were just the normal silkworms you get with crooks anyway no matter whether it is infested or not. Maybe infested infested oak trees drop string. No idea. Just a random guess.
I would assume that the string drop is default, but maybe you only tested it on maple.
But there is no need for string to drop anyway. There are enough spiders around.
My last attempts on the Savanna map failed miserably. I usually do well for some time and then I get careless and that is when bad things start to happen.
Yesterday I was like 1200 blocks away from spawn after my initial exploring and then I died. Had even found 2 or 3 apiaries. When I respawned at spawn I got immediately attacked by a large zombie and a baby zombie and died. Guess what happened after I respawned again?