To add to this, I also discovered that tftpd-hpa only logs to syslog, not stdout/stderr, even when running in foreground mode. This is why no matter what I set the verbosity flag to, I was not seeing any output.
Since I am running it in Docker and want logs to go to stdout/stderr so I can view them with the docker logs command, I had to run a syslog client in the Docker container as well to receive the messages tftpd-hpa was sending.
I found an example online that I borrowed, which used syslog-ng and configured it to write everything to stdout.
With this running, the logs appear:
$ docker logs abc123
Jul 3 09:37:57 vorticon syslog-ng[7]: syslog-ng starting up; version='3.30.1'
Jul 3 09:39:00 host in.tftpd[11]: remap: input: test2a.txt
Jul 3 09:39:00 host in.tftpd[11]: remap: rule 0: rewrite: 192.168.0.1/test2a.txt
Jul 3 09:39:00 host in.tftpd[11]: remap: done
Jul 3 09:39:00 host in.tftpd[11]: RRQ from 192.168.0.1 filename test2a.txt remapped to 192.168.0.1/test2a.txt
Jul 3 09:39:00 host in.tftpd[11]: sending NAK (1, File not found) to 192.168.0.1