Board Thread:General Discussion/@comment-28109177-20180327021938/@comment-24152851-20180327051047

Not a plot hole exactly. Just very poorly explained through their goto BS tine travel exokanation: time remnants.

Essentially, how the show gets around the paradoxes is that the Speed Force will occasionally preserve a speedster's previous timeline in order to allow the current one to exist.

So we all know that Eobard was erased from existence ever since Eddie killed himself. But the only reason why Eddie killed himself to begin with was to stop Eobard. Hence, Eobard himself caused Eddie to kill himself and erase him. Which means, without Eobard messing with the timeline to begin with, his erasure was never possible. Uh oh. There's a problem. So what the writers/Speed Force did was allow Eobard (and Eobard alone) to continue existing as a timeline remnant, where he gets to keep his personal history from the timeline that no longer existed, in order for the new timeline to exist.

Now back to Flashpoint/Legion Eobard. The time remnant business comes into play again. Eobard was supposed to kill Nora Allen, lose his speed, and get trapped in the present as Harrison Wells. But thanks to Barry screwing with time, one version of Eobard managed to avoid that and broke free of his predestined doom, continuing to exist as the time remnant of the Flashpoint timeline. The original timeline where Eobard kills Nora and becomes Wells is still there (albeit slightly changed), while the Flashpoint remnant Eobard goes and cause trouble for the Legends.

And that's how the extremely BS time travel mechanics works on The Flash. Hope that helped.