When you mix RAM here are the possible outcomes.
- You put the same brand and model RAM with the same speed and everything works fine. XMP works and no issues arise.
- The system doesn't even boot.
- The system boot but you have instability issues, crashes and XMP profile doesn't turn on and requires you to manually set everything.
Mixing RAM is a luck thing even if you buy the same brand, model and speed RAM. This is why people always buy the amount of RAM they want total as a 2 stick RAM Kit and you just remove the old RAM.