Hyper-V enhance mode 19.04
Switching to the Hyper-V optimized kernel fixed this for me:
sudo apt install linux-azure
(I also removed the old kernel with sudo apt autoremove --purge but that might not be important.) Then a restart is necessary. When it came back up it went into enhanced mode.
I'm on Windows 10 version 1909 build 18363.535. I installed Ubuntu 19.04 with "Quick Create" and have not upgraded it to 19.10.