So I was able to fix my problem. The WebUI link generation is in /usr/local/emhttp/plugins/dynamix.docker.manager/include/DockerClient.php. I just wrote a user script that patches the link generation that just allows a manual, user-specified WebUI if all other methods fail. I was going to try to put in a PR to contribute the fix upstream at github.com/limetech/dynamix.git, but that repo hasn't been touched since 2016. Also, the version there is different from the version on 6.9.2. I'm not sure which is more up-to-date as I don't have the date on the version in 6.9.2, though interestingly enough, the one on github looks like it wouldn't have this problem. Did limetech take the code to closed source or is there another place that the code is hosted?
