https://man.freebsd.org/cgi/man.cgi?query=mdmfs&sektion=8&apropos=0&manpath=FreeBSD+14.0-RELEASE+and+Ports
Say:
> Create and mount	a 16 megabyte malloc-backed file system	on /tmp	 using
>       the  /dev/md1  device;  furthermore,  do	not use	soft-updates on	it and
>       mount it	async:
>
>	     mdmfs -M -S -o async -s 16m md1 /tmp

But soft updates enabled and no "async".


# mdmfs -M -S -o async -s 16m md1 /media

# mount
/dev/md1 on /media (ufs, local, soft-updates)

# tunefs -p /dev/md1
tunefs: POSIX.1e ACLs: (-a)                                disabled
tunefs: NFSv4 ACLs: (-N)                                   disabled
tunefs: MAC multilabel: (-l)                               disabled
tunefs: soft updates: (-n)                                 enabled
tunefs: soft update journaling: (-j)                       disabled
tunefs: gjournal: (-J)                                     disabled
tunefs: trim: (-t)                                         disabled
tunefs: maximum blocks per file in a cylinder group: (-e)  4096
tunefs: average file size: (-f)                            16384
tunefs: average number of files in a directory: (-s)       64
tunefs: minimum percentage of free space: (-m)             8%
tunefs: space to hold for metadata blocks: (-k)            40
tunefs: optimization preference: (-o)                      time
tunefs: volume label: (-L)                                 

# mount -v
/dev/md1 on /media (ufs, local, soft-updates, writes: sync 2 async 0, reads: sync 3 async 0, fsid 1572526616e23f74, vnodes: count 2 )

# uname -a
FreeBSD rimwksv 14.1-STABLE FreeBSD 14.1-STABLE RIM_WKS amd64


Also, IMHO, -t should be forced for all memory backend mount points.