Catalina partition turns into type FFFFFFF/unbootable and unmountable "disk0s2" after Mavericks install I'm having a similar issue with this (MacOS partition formatted FFFFFFF etc after bootcamp install), but I got confused while reading the comments.
I thought it would be a great idea to dualboot Catalina and Mavericks on my laptop, and I already had a Bootcamp-ed Windows 10 partition that was unused, so I wiped that, installed Mavericks, all went fine until it restarted and my Catalina partition didn't show up in the boot picker.
I assumed it was just simply because Mavericks has no idea what APFS is - it just showed up in Disk Utility as &quot;disk0s2&quot;. I created a Catalina installer, booted into its disk utility and it doesn't even show up, until you go to the partition menu.
As I said earlier, I got lost unfortunately with all of the guides. Any help is greatly appreciated, especially for keeping the data in disk0s2 intact.
This is what gpt -rf show disk0 gives me:

It's this classic FFFFFFFFF partition type again.
Boot to the Catalina installer and enter the commands shown below.

Note: The values 409640 and 349609376 were taken from the output posted in the question.

gpt -f remove -i 2 disk0
gpt -f add -i 2 -b 409640 -s 349609376 -t apfs disk0