Sad to hear it didn’t seem to work. It makes me think of how some browsers cache redirects, like this thread, and on Chrome and Safari (maybe more – I didn’t keep googling).
Suppose your browser caches the original redirect from B.com to A.com. Now you switch the redirect in your Nginx conf, making B primary. Then you go test whether B is working as primary but the browser has cached info saying redirect B to A. So the browser sends you to back to A where you’ve created the new redirect to B, ad infinitum.
If that’s what’s happening, it probably wouldn’t be resolved by rebuilding the droplet and reprovisioning from scratch. You can clear your own browser’s cache but I haven’t researched this to know whether there is any way to do cache busting for other regular visitors to your site. 