Disk0s2 showing up in Disk Utility

Please try the following:

1. Temporarily disconnect your external drives.
2. Run the Terminal app, and enter: diskutil list <enter> and post a screen shot of the results.
3. Next, enter: diskutil info disk0s2 <enter> and post a screen shot of that as well.

Hmm, everything seems normal, except for disk0s2 which is identifying itself as a HFS partition. This shouldn't exist if both drives, that make up the Fusion Drive, are formatted for APFS. Below is a screen capture from my 2014 Mac mini. In my case disk0 is an external WD My Book Pro, do macOS assigned disk1-3 for the Fusion Drive. This partition did not even exist prior to macOS Mojave reformatting my Fusion Drive from HFS to APFS during an upgrade.

Ref: About Fusion Drive, a storage option for some Mac computers - Apple Support

At the same time, it doesn't appear that you have a "split" Fusion Drive. I am assuming that you cannot separately access each of these drives in Finder ... correct? If you can, then see if the following article can help:

• How to fix a split Fusion Drive - Apple Support

Honestly, at this point, I don't know as I've never come across something like this before.

I would think you basically have two choices, going forward:

1. Since everything is working, leave it as is, OR
2. Continue to troubleshoot/resolve it, if it "bothers" you. With this option, I suspect that you can either make a Genius Bar appointment at your local Apple Store or, maybe, someone here that has come across this will chime in to offer you a solution.