With all versions of Hyper-V, including Server 2019, Linux video resolution is limited to 1152 x 864, and it cannot be changed. Although this has no impact on Linux performance as a server--and is a non issue with GUI-less installs--it is still an annoyance for those with GUI.
When Linux with a GUI is installed in Oracle VirtualBox or VMware Workstation (or vSphere), there is no limitation on resolution.
MSFT, do you plan on fixing this?