Usually when OBS disconnects and reconnects, Twitch will automatically make a new broadcast. Can a speedrun still be valid then?

Depends how much time is missing from the run. If it’s a few seconds, it’s usually fine. If it’s multiple minutes, it’s usually not fine.

At the end of the day, there’s not global rules. Ask the mods of the game you were running what their rules are.

It depends on the game and how strict its moderators are. I wouldn't verify a run with a significant cut in the middle on any of the games I moderate, and I know a lot of moderators that also wouldn't. But then there's also games that don't require video proof at all, so on any of those you'd be fine.

Side note, if possible always locally record your runs in case stuff like this happens.

A local recording is always preferable. I don't trust twitch to begin with and I trust my internet connection even less.

Up to the game moderators to decide if too much footage is missing. It's at their discretion.

There's some games I follow where any footage being left out at all would be an instant reject. Utilize local recording, there's really no excuse for this stuff.

Do a local recording. It'll be better quality probably, and won't have random dropouts.