Unable to prevent APFS partition from mounting automatically in Mojave (10.14.5)
"Educated" guess:

In your case FileVault unlocking is managed by the preboot APFS volume (disk1s2).

Your specific APFS container configuration implies that preboot manages booting to macOS 10.14.5 as well as macOS 10.15 beta.

At this boot stage disk1s1's /etc/fstab is probably not yet involved and thus entries won't work.



If you really want to disable disk1s6/disk1s7, you probably have to rename one of the two folders with UUID names (one for macOS 10.14.5 and the other for macOS 10.15 beta) in preboot. The folder names correspond with the APFS boot volume UUIDs you get with diskutil ap list.

You have to rename the one which is identical with the UUID of macOS 10.15 beta's boot volume.

Not tested - absolutely not recommended!



BTW: I tried to find some kernel flag to add to /Volumes/Preboot/[UUID]/Library/Preferences/SystemConfiguration/com.apple.Boot.plist to prevent FV unlocking but failed.