Issue with TFTP and IPTables and (s)-NAT
You probably need to load tftp NAT helper as tftp is one of these protocols which don't play nicely with NAT.
Try modprobe nf_nat_tftp.
Note:
The above info is from a system using nftables, not iptables. Your mileage may vary. In any case try to find it under /lib/modules/<kernel-version>/kernel/net/ directory, likely in a subdirectory netfilter on top level or under ipv4 or ipv6 directories.