Okay, I tried and it worked for me.
First, with init. That is, it’s a fresh container that did not launch yet.
$ lxc network create WEBDNS ipv4.address=10.99.10.1/24 ipv4.nat=false
Network WEBDNS created
$ ps ax | grep 10.99.10.1
18691 ? S 0:00 dnsmasq --strict-order --bind-interfaces --pid-file=/var/snap/lxd/common/lxd/networks/WEBDNS/dnsmasq.pid --except-interface=lo --interface=WEBDNS --quiet-dhcp --quiet-dhcp6 --quiet-ra --listen-address=10.99.10.1 --dhcp-no-override --dhcp-authoritative --dhcp-leasefile=/var/snap/lxd/common/lxd/networks/WEBDNS/dnsmasq.leases --dhcp-hostsfile=/var/snap/lxd/common/lxd/networks/WEBDNS/dnsmasq.hosts --dhcp-range 10.99.10.2,10.99.10.254,1h --listen-address=fd42:da26:23c4:bfa5::1 --enable-ra --dhcp-range ::,constructor:WEBDNS,ra-stateless,ra-names -s lxd -S /lxd/ --conf-file=/var/snap/lxd/common/lxd/networks/WEBDNS/dnsmasq.raw -u lxd
18710 pts/7 S+ 0:00 grep 10.99.10.1
$ lxc init ubuntu:18.04 c1
Creating c1
$ lxc network attach WEBDNS c1 eth0 eth0
$ lxc config device set c1 eth0 ipv4.address 10.99.10.42
$ lxc start c1
$ lxc list c1
+------+---------+--------------------+-----------------------------------------------+
| NAME | STATE | IPV4 | IPV6 |
+------+---------+--------------------+-----------------------------------------------+
| c1 | RUNNING | 10.99.10.42 (eth0) | fd42:da26:23c4:bfa5:216:3eff:fe11:6a97 (eth0) |
+------+---------+--------------------+-----------------------------------------------+
Now, let’s try the same with a container that has been launched with the default profile.
$ lxc launch ubuntu:18.04 c2
Creating c2
Starting c2
$ lxc stop c2
$ lxc network attach WEBDNS c2 eth0 eth0
$ lxc config device set c2 eth0 ipv4.address 10.99.10.82
$ lxc start c2
$ lxc list c2
+------+---------+--------------------+-----------------------------------------------+
| NAME | STATE | IPV4 | IPV6 |
+------+---------+--------------------+-----------------------------------------------+
| c2 | RUNNING | 10.99.10.82 (eth0) | fd42:da26:23c4:bfa5:216:3eff:fe00:de0b (eth0) |
+------+---------+--------------------+-----------------------------------------------+
This also works.
Still, there might be a case where it does not work. You mention that you are using a profile for this containers. It could be indeed a profile issue when you attach to a container.
Can you post a full test case (as I do above) which also shows how such a minimal profile is created?