Don’t use FFSplit, seriously. I know that’s probably not the answer you want but there’s a reason the FFSplit forums are dead. OBS is just the way to go for broadcasting and recording and since OBS Studio is multi platform there’s no reason not to use it. I’d make the switch to that and see if that solves your problem.

What is your capture card, or is it off the same computer?

I will echo @Timmiluvs and suggest using OBS as it is actively supported, but if the issue is not FFSplit then this is merely a suggestion to take or leave at your pleasure.