i have no idea. but your not being clear. do you mean "Disk Utility" mounts the drive without asking for a pass? But your problem is it's not auto mounted, so that issue is not yet arrived, because your first problem is it's not automounted.
/Users/you/Library holds personal settings, app data specific to you (perhaps that disk you made encrypted - the auto settings were in Library and are not part of upgrade). Library settings are not always upgraded perfectly because apps have tons of options and some old options are not supported and some option combinations are simply not predictable (for 75 options calculate 75! (75 factorial) and see how many loose ends there can be).
your first problem is to find out if this is a disk setting (to change on that disk - perhaps in extended partition type), an OS default setting, or a user/Library stored setting
(the below wrong for your problem, but an example. also note 'defaults' can list all categories / items that have settings on the system)
$ defaults write com.apple.finder AppleShowAllFiles TRUE
(here we see that mojave has a breadth of tools from the Terminal app. i have no time to chase down this issue though. open terminal:)
$ man man
$ man diskutil
hdiutil(1), mount(8), umount(8), diskmanagementd(8),
diskmanagementstartup(8), diskarbitrationd(8), corestoraged(8),
fdesetup(8), ioreg(8), newfs_hfs(8), fsck_hfs(8), authopen(1),
hfs.util(8), msdos.util(8), ufs.util(8), drutil(1), vsdbutil(8)
i assume diskarbitrationd is interesting and should be running (but ends up not being the issue)
I would instead check Launchpad->Console, which is very thorough, and see what messages arise concerning the disk when it is mounted. If there are no "hang ups" or errors, then you can assume mounting is normal and your issue is that the disk is simply not selected for automount.
did you try?
it could simply be that your disk has a mount point to a directory that autofs no longer auto mounts in mojave
on the other hand, if you have a backup, your intuition that it wouldn't be a bad idea to "re-create" the encryped drive. perhaps the filesystem is different and the old one, if encrypted, is not automounted, may be right
I see advice on how to prevent automount but not how to "re-enable it" since it's rather automatic.
Just because it's hard to find advice on it may be reason enough to do it "the long way".
I really can't help more because i don't have an encrypted journal (which mac format?) drive to run tests on. sorry.