A new motd script will not run

I created a new script 99-wlan0-RWB in /etc/update-motd.d which is world executable like all the others. It works when run on its own or via run-parts, but is not run when logging in: what's wrong? (I have the same problem on a Raspberry Pi where _none_ of the scripts in update-motd.d run.) (Nothing in /var/log/syslog.)

$ ssh root@mini31
Linux mini31 4.19.0-18-amd64 #1 SMP Debian 4.19.208-1 (2021-09-29) x86_64
External IP: xx.yyy.zzz.www
 ______________________________________
/ 1st graffitiist: QUESTION AUTHORITY! \
|                                      |
\ 2nd graffitiist: Why?                /
 --------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
df: 43G (81%)
    O2: 1.65 GB / 3.3 GB
Lebara: 6.49 MB / 66 MB
up: 10 days, la: 1:0.11, 5:0.14, 15:0.10
mssql tempdb.mdf is 515M
CPU temp.: +52.0
No mail.
mini31 # cd /etc/update-motd.d/
mini31 # ./98-data
df: 43G (81%)
    O2: 1.65 GB / 3.3 GB
Lebara: 6.49 MB / 66 MB
up: 10 days, la: 1:0.18, 5:0.15, 15:0.10
mssql tempdb.mdf is 515M
CPU temp.: +50.0°C
mini31 # ./99-wlan0-RWB
wlan0 is up
mini31 #