All of my companions are walking off into some random direction and when I fast travel they are automatically dismissed.

It started with Dogmeat when he just wandered off as I was defending an outpost.

I had to get another companion to come with me to be able to send him back to whichever settlement I needed him in.

But now even that doesn't work and after a while they are nowhere to be found.

No they are not going towards their homes because Piper exited Diamond City (by herself) and just started walking out of the area using a bridge and then I'm 99% sure she proceeded to walk South- East (in the Gunner Plaza direction, which I have recently cleared out).

I can fix it by accessing an earlier save but I honestly don't want to replay the same stuff for the next 12 hours just to find out it'll happen again.

Note it only happens when I ask them to travel with me, if I make them stay at a settlement they stay there. Please help.

I had that same problem earlier on in the game with dogmeat vanishing all the time then it seemed to correct itself after the kellog questline.

Try proceeding a bit further through the main questline and see if that has an effect. It could also be a residual command the Ai is trying to perform hence why they keep leaving the area when you fast travel. Either follow them to where their going or go about doing some tasks while the Ai completes it's terrible programing and resets.

I completed the main questline so I don't think that's it... but yeah I will follow them although I'm really not looking forward to travelling across the whole Commonwealth at walking speed

Thanks for the help haha