Please pull both sticks out and tell us their part numbers.
If they don't have similar timings and what not, it could be causing issues. Just because it used to work on one platform doesn't mean it will on another. I'd honestly recommend selling them and getting a proper dual channel memory kit of 3200Mhz memory instead
Do check if there's a bios update available for increased stability, though since you have a 3000 series Zen2 CPU and a B450 motherboard, you likely already have the latest one.