This seems to be the correct answer - it appears to be the only way of getting a properly working install of OpenJDK for Windows that will actually be found by other apps - but it's a third-party build of OpenJDK backed by most of the major Java supporters except Oracle.
It is not backed by Oracle because OpenJDK's developers, who are 90% Oracle employees, do not use this build and don't know how it's made (it doesn't just use the OpenJDK make files), but rather the "official" OpenJDK build at http://jdk.java.net/. Similarly, the "official" OpenJDK build for 8 and 11 is the Red Hat one here (hosted on Adopt, but not an Adopt build): https://adoptopenjdk.net/upstream
These are the only builds for which you can directly report OpenJDK bugs against. This is not because of some conspiracy, but merely because most organizations that provide JDK builds (like Amazon or Adopt) do not contribute much or at all to OpenJDK -- usually because they don't have the resources or the knowledge, and in some cases because they don't want to.
Because the responsibility of developing and maintaining OpenJDK -- in all builds -- mostly falls to Oracle developers, we can only "back" builds we actually know. If you find a bug in Adopt, you'll need to report it to someone who will first reproduce it on an official build and only then report it to OpenJDK, where, in all likelihood, an Oracle developer will address it. Other companies can back Adopt because they're not the ones who maintain the project, anyway, so they bear little or no responsibility. We're happy that OpenJDK is doing so well, but with limited resources, we can only focus on builds that are under our control.