I have no explanation for your reported behavior, and I could not quite work out whether it is the server or one of the clients that had lost it's IP address. I just wonder whether you had considered to use
udhcpd on the server and leave the clients on the default DHCP configuration.
I've got to admit I have not tried it myself and would probably need to read a web page like
this to work out the config file. But this might be worth a try. If you consider this approach you could run 'udhcpc' and 'udhcpd' with the '-S' option. Obviously in this case you need to run 'sudo syslogd' beforehand (or use the 'syslog' boot code).