I'm also having this issue (or a very similar issue), and no settings will fix it, it's a bug for sure
I have a DNN website instance running with two portals (portalId 0 and 1)
Both are multilanguage (en-us and pt-br. pt-br is the primary language.)
Both are configured to Canonical redirect mode.
Portal Id 0 works in both languages with dedicated portalAlias for each language. (http://mywebsite and http://mywebsite/pt-br for pt-br cultureCode and http://mywebsite/en-us for en-us cultureCode)
Portal Id 1 just dont work the same. No matter how I set the redirect mode (canonical or none) it won't work. I just can't navigate to the en-us version of portal id 1. (pt-br is the primary language in both portals)
I found a related bug listed on
https://dnntracker.atlassian.net/brow... and
https://dnntracker.atlassian.net/brow... but both were closed, I don't think who closed it DID understand what was happening.
I will try later to debug the DNN source code to try find whats happening inside, anyone can point me the related files so I can quick jump to it ?