I use a built-in screen recorder on my computer and when I review footage I notice that the livesplit timer I use wasn't present. should I use different recording software?

Yes, I would always recommend using OBS unless there is a very specific reason why it doesn't work.

Yeah I'd just recommend using OBS since it's not hard to use and it's very customisable but I will note that you don't need the timer on the video as the mods can retime the run but it's obviously preffered.

Built-in recorders and just recording software, in general, tends to grapple onto a running application to capture, sometimes from a specific engine, so it prioritises the application for recording and sets this stuff back

Doesn't always matter, you can add-on the timer after the gameplay, if you don't want to then use Livesplit for the timer during the gameplay, and use another app to record.

Game mods should be manually retiming your run anyhow, so screen timers aren't necessary... except if the game requires them for validity. So make sure you check the game's rules.

Do you actually know of any leaderboards that require runners to add an external timer?

@Pear Here's a few: Resident Evil 2 when played on PC Spongebob Battle for Bikini Bottom Rehydrated for sufficiently fast times The Vanishing of Ethan Carter Path of Exile