Subject: How to fix a split Fusion Drive. Do new internal NVMe and SATA SSD need to be formatted APFS prior to combining?

Here is: How to fix a split Fusion Drive - Apple Support

OEM Fusion drive contains Apple's NVMe SSD and another large HDD..

Fusion drive - to my experience was more of a pain for current day of use.

You indicated your Fusion drive contains NVMe and SATA SSD?

That means you or others put a 3rd party SATA SSD for the original HDD?

If so, I will highly suggest you just use the large SATA SSD for the system - do not bother to "fuse" both drives again..

By fusing 2 SSDs - did cause me countless system hiccups (I have 2015 5K iMac).. In the mean time, Apple's NVMe SSDs are quite small that make the Fusion drive quite unstable.

If you insist to do so, you do have to use terminal command.. (Google it) -- I did use a USB boot drive to achieve that, and did format them into APFS..

Good luck