@cshoredaniel is correct.
See this comment from bep:
Using HUGO_MODULE_REPLACEMENTS is the same as manually editing the path in your module config in config.toml – which is not the same as a replacement inside go.mod
In your site configuration, change this:
theme = 'git.sr.ht/~skyfaller/maxethics-hugo'
to this:
[[module.imports]]
path = 'git.sr.ht/~skyfaller/maxethics-hugo'