I tried to make a custom login form based on https://premium.wpmudev.org/blog/customize-login-page/ with a success. However, I am struggling that the register link (or sign u p) that it kept going redirecting to the root url directory. I disabled my my-login() function. I see I have the same problem when I used a login theme plug in. All ran fine on the subsite but the register page always go to the root URL. I am not sure what I could do in my multisite web.
Your insight will be appreciated. I have grant the access just in the case.