help gt seeds aswell as bud what is going on

hi i am growing in a nft system. ive gt 20 plants in a 6ft double channel tray. i noticed about a week ago that one of the plants closest to the resovoir was turning into a different colour. it looked like a bit of rot. i dont know if it was. the rest of the plants seemed fine so i thought id cut the bad plant out. i noticed that the bud was dry straight away. there was no stickiness or anything. it was bone dry. but as i was crumbling it i noticed there was seeds in the bud. what is going on here. thee seeds i bought were from marijuana-seeds.nl. they were femenized jack herer. i dont think it is real jack tho cus ive grown jack brfore and it was alot better and that was from cuttings from a friend. im in the 8th week of floweing. will that plant affect the rest of them and why is there seeds growing aswell as bud. i have not noticed any seeds on the rest of the plants. but i didnt notice them on da bad one until i crumbled the weed.