26 May 19 20:04, you wrote to all:

 TL> I have a Debian 9 netbook here.  Because I have an unusual setup here, I
 TL> need to use manual IP address configuration.  I used the default network
 TL> manager to setup the static IPv4 and IPv6 addresses on the wifi
 TL> interface.  When I start the system, all works fine, and the correct IPs
 TL> and routing information as I setup via the GUI are in place.

 TL> However, when there's a network upset (most probably due to the router
 TL> rebooting, or some other wifi disturbance), the static IPs are gone and
 TL> are replaced by a DHCP assigned IPv4 address.  This causes a number of
 TL> issues, breaking connectivity with the BBS that is running on the
 TL> system.

 TL> In the good old days, I'd have done a total manual configuration, and
 TL> this would have stuck, but I decided to use the tools provided for 2
 TL> reasons:

 TL> 1.  To work with the new ways of doing things, rather than figting the
 TL> system.

 TL> 2.  To simplify the configuration of the wifi (i.e. not having to mess
 TL> around with wpa-supplicant directly).

 TL> Anyone seen this behaviour?  And better still, anyone have a solution?

Have you considered assigning the netbook a static IP address using DHCP?

I believe most modern routers support this, though I prefer to disable my 
router's DHCP server and run my own dnsmasq service on a separate machine.

dnsmasq has some useful options in dnsmasq.conf:

# Always give the host with ethernet address 11:22:33:44:55:66
# the name fred and IP address 192.168.0.60 and lease time 45 minutes
#dhcp-host=11:22:33:44:55:66,fred,192.168.0.60,45m

# Give a host with ethernet address 11:22:33:44:55:66 or
# 12:34:56:78:90:12 the IP address 192.168.0.60. Dnsmasq will assume
# that these two ethernet interfaces will never be in use at the same
# time, and give the IP address to the second, even if it is already
# in use by the first. Useful for laptops with wired and wireless
# addresses.
#dhcp-host=11:22:33:44:55:66,12:34:56:78:90:12,192.168.0.60

# Give the machine which says its name is "bert" IP address
# 192.168.0.70 and an infinite lease
#dhcp-host=bert,192.168.0.70,infinite

--- GoldED+/BSD 1.1.5-b20180707