Linux machines with multiple network interfaces are unreliable as TFTP servers. This issue has been outstanding for a long time, without any resolution. The patch attached to the Debian bug fixes the problem for an old release of tftpd-hpa, but does not apply cleanly to recent releases.
Recent releases of dnsmasq contain a TFTP server which does not have this problem. While this doesn’t solve every case, it provides a tidy solution for a machine which provides BOOTP and TFTP services to several subnets.