exfat fsck crash on macos 10.14.4. Any workaround or fix?

This is not a problem with the drive. Newly-formatted drives get the same fsck_exfat error. It is a problem with fsck_exfat on versions of Mac OS X newer than Snow Leopard.

As man fsck_exfat says:

The fsck_exfat utility first appeared in Mac OS X 10.6.3.

I don't know how you can bypass the required automatic fsck in macos 10.14.4. It is fsck_exfat -q <disk> that quickly checks to see if the volume was "cleanly unmounted". Perhaps that is being automatically run. I don't know if booting into safe mode would stop the automatic check.

Maybe try downgrading to the version of fsck_exfat that comes with Snow Leopard and see if it still reports an error with fsck_exfat_cache.c. which fsck_exfat reports that it is located at /sbin/fsck_exfat.

If you are concerned about the external HD, use fsck_exfat that comes with an updated version of Mac OS X Snow Leopard (Mac OS X 10.6.8), or try a free version of Linux as jaume commented. You can buy a Snow Leopard retail disk. With CentOS, install the exfat-utils and fuse-exfat EPEL packages.

This is the kind of output you should be receiving:

$ sudo fsck_exfat /dev/disk3s1
Password:
fsck_exfat: Opened /dev/rdisk3s1 read-only
** Checking volume.
** Checking main boot region.
** Checking system files.
** Volume name is [volume name].
** Checking upper case translation table.
** Checking file system hierarchy.
** Checking active bitmap.
** Rechecking main boot region.
** Rechecking alternate boot region.
Mark volume clean? n
** The volume [volume name] appears to be OK.

Read Using exFAT as a common filesystem at Galaxy Verge.