Starting from 10.5.11 (and any other release carrying the same version of the affected code), wsrep_sst_mariabackup does not work on FreeBSD. There are two issues affecting us:

• reliance on --tmpdir option to mktemp
• is_local_ip always returns true in a jail

The former makes the SST script fail because of invalid option.
This is easily (and probably compatible with Linux) fixed by removing the --tmpdir option and rather prefixing mktemp with 'TMPDIR=$tmpdir ;' inside the same $() around line 935 of wsrep_sst_mariabackup:
< xtmpdir=$(mktemp '-d' --tmpdir "$tmpdir")
> xtmpdir=$(TMPDIR="$tmpdir" ; mktemp '-d')

The latter I had to hack by removing the commonname option passed to socat on line 387 of the same file:
< CN_option='localhost'
> CN_option=''

This is obviously not a fix, as I don't fully understand what's going on - but I did not have more time to spend on this as our cluster was down at the time.