I just updated from 3.10.0 to 3.12.1 and gem resolution is problematic. It wasn't perfect in 3.10.0 either (if you want a seperate ticket, let me know).
I have a rubygems group repository called 'gems' that contains hosted and proxied repositories. The following behavior was determined by running "bundle update --verbose" with bundler version 1.16.1, ruby 2.5.1.
In 3.10.0, the first request to https://nexus.example.com/nexus/repository/gems/versions yields a 400 with no error in the nexus.log. This causes a second request to https://nexus.example.com/nexus/repository/gems/api/v1/dependencies, which works correctly.
In 3.12.1, https://nexus.example.com/nexus/repository/gems/versions leads to a 400 and https://nexus.example.com/nexus/repository/gems/api/v1/dependencies gives a 404 (again, nothing concerning in nexus.log). This causes bundler to download the specs and check each gemspec, which is a slow process.
The ideal behavior is that bundler can use https://nexus.example.com/nexus/repository/gems/versions.