Users are discussing issues with Discourse redirecting to the root directory instead of a subfolder after login, despite following setup guidelines. Science_Geeks reports the problem after migrating from a subdomain to a subfolder for SEO reasons. pfaffman and RGJ clarify that subfolder vs. subdomain has minimal SEO impact, citing Google’s stance that it doesn’t matter. RGJ references Google’s Gary Illyes, who has stated that subfolders offer no benefit over subdomains. Jagster agrees, dismissing SEO as a valid reason for using subfolders. Magnetidog confirms the issue persists after an update, indicating a possible bug.
I configured my discourse forum to be in a subfolder(migrated from subdomain to subfolder which entailed moving from hosted by discourse to self hosted) by following the guidelines on doing that. Now everything works, except for the fact that if I login(regular login), it logs me in and redirects to the root directory instead of the subfolder.
E.G I login @ www.website.com/forum/login and it redirects to www.website.com instead of www.website.com/forum after login.
Most everyone thinks a subdomain is better, but you do you.
Did you follow Serve Discourse from a subfolder (path prefix) instead of a subdomain
Yes I did
Also, using subfolder for seo reasons.
Two more people having the same issue here Serve Discourse from a subfolder, Login redirects to root path
There is no seo reasons 
I was informed that having forum in root domain level will be good seo-wise for my main service(example.com). Are you saying that is false?
There has been a lot of discussion about this. My personal opinion is that it indeed does not matter.
And mostly people stopped discussing this some years ago.
The likelihood that you’ll get a significant seo drop of because all of your content has moved is pretty assured. It’s unlikely that it’ll be better after you have recovered to your current levels in some months.
Same issue as @Science_Geeks reported after updating to the latest build.