I have no insight into why that split recording setting is in Advanced, though I can guess, that in general, the developers don't want to expose features that will cause inexperienced/newbie users more hassles. Whether that applies to this or not, I can't say.
But for you, it should be easy to simply fix your Advanced settings
1. I believe there is a way to reset those Advanced Recording settings back to Default, but I don't have that info in front of me (if such is an option, without resetting the entire Profile. So, your task is to look into this.
2. My recommendation - (as searching in these forums for the recent post that actually spelled out the underlying Simplified settings may take longer)
- record using Simplified, then review the OBS Studio log, which will list the details
- Now, go to Advanced and use same settings. I would assume you changed something, intentionally or not, in the Advanced settings causing your issue. Use the settings you find in the log for Simplified as a starting point.
Beware YouTube video with OBS Studio settings recommendations from clueless fools who don't provide context, and whose advice is counter-productive. There are plenty of people who think they know what they are doing, but don't. Following their advice will cause problems.
Running your logs through the Analyzer (see my .sig) may also provide other insight
