the humidity in the tent is between 60 and 70%, even though I don't have a humidifier. I don't know why. Should I lower it or how can I lower it?

Autos will flower when they want to. As far as I'm aware, there's no trick to prolong their been period. I have however found if you top them and doesn't seem to effect them as much as you'd think.

FAE, Fresh Air Exchange is normally how you lower RH. however if the fresh air your pulling into the tent has a similar RH. Then it won't lower it. Tents RH tends to run high as the plants are letting off a lot of mositure in the small space.

Plants 'transpire' through the leaves. The larger the plant, the more moisture it will pump into the atmosphere. You can extract air to an area where it can't cycle back into tent's room, or you can buy a dehum. The first requires ambient RH to be equal or lower than your tent's target RH and if that is not the case, extracting won't do shit but move air that has too much moisture from one area to another... not effective. You can also choose to grow during seasons with low RH and possibly avoid needing a dehum. Dehums also add heat. It's much cheaper to add moisture than to take it out of the air. If it is 8 weeks above ground, good chance your 'autoflowers' are actually photoperiods. This happens from mispackaging or pollen contamination. It should not happen often. If it happens repeatedly, stop buying from that source. Expect them to at least double in size. If this is too much, prune them back and give 12hours of light / 12 hours of darkness. May want to prune and let it heal for ~5 days before flipping to a photoperiod flower schedule. If you go to 12/12, stick to it. You need 150% more light to maintain same dli over 12 hours vs 18 hours, otherwise you are just providing 2/3rds the dli.

Autos will bloom when they want some autos are fast and small and others can get huge