When you move/create form files in subfolders (relative to project root) convetion based system fails to find associated resx file, because it clearly mangled the filename (see build error)
if you disable EmbeddedResourceUseDependentUponConvention in csproj this error goes away.

this is likely related to #4597

Steps to reproduce

sample project
WindowsFormsDependentUponConventionTest.zip

Expected behavior

assembly builds

Actual behavior

build error:

Error MSB3041: Unable to create a manifest resource name for "Forms\Form1.resx". Could not find a part of the path 'C:\Users\Vladislav Richter\source\repos\WindowsFormsDependentUponConventionTest\WindowsFormsDependentUponConventionTest\Forms\Forms\Form1.cs'.
C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\MSBuild\Current\Bin\Microsoft.CSharp.CurrentVersion.targets 100

Environment data

VS 16.3.0 Preview 3.0
.NET Core 3.0 Preview 9
WinForms project